Stepping on stage in Barcelona recently for the opening night of U2's 360° tour, Bono could be forgiven for heaving a discreet sigh of relief. Though U2 safely remain rock's biggest draw (their latest LP shot to number one in 30 countries) the past year and a half hasn't been the smoothest for the band.

There have been embarrassing album leaks; critical and commercial knock-backs and, perhaps most hurtful of all, renewed accusations of financial hypocrisy. In a rare lapse, Bono even managed to call Chris Martin of Coldplay a 'wanker' on live radio. In the wake of such controversies, the first night of a six-month stint on the road -- to say nothing of the sweaty love of 80,000 Spaniards -- must have felt like a refreshing splash of water across the face.

So how bad have things been for U2? By anyone else's standards: not very. While 80s peers such as REM, Simple Minds et al slip ever closer towards middle-age bloat, U2 continue to make music that matters -- both to themselves and their millions of fans. Embarking on a stadium jaunt in the midst of the biggest financial crisis since the depression, they have sold out the bulk of their dates (tickets have been snapped up particularly quickly in their spiritual homeland of the United States).

In a few months, Bono and Edge will begin work on a Broadway musical about Spiderman -- an undertaking from which they will, in all likelihood, emerge with dignity intact, if not enhanced (try to picture Radiohead writing a musical about Wonderwoman -- and feel the despair well in the pit of your stomach).

At the same time, however, it's not strictly accurate to say everything has been proceeding according to plan. While you won't catch them moaning about it -- U2 have always kept their grumblings firmly out of public view -- there is no doubt that the band has suffered a series of not-insignificant knock-backs lately.

With the international credit markets in meltdown, it is unclear when, if ever, the Norman Foster's U2 Tower will be built at Dublin's docklands. A 2006 decision to relocate their tax affairs in the Netherlands, in order to benefit from legal loopholes, has, meanwhile, come under renewed attention: ought we really take lessons in how to be better people from individuals who off-shore their wealth in order to wriggle out of paying income tax at home? And what are we to make of Larry Mullen Jr's recent -- and highly uncharacteristic -- outburst to the effect that Irish begrudgery is making the country an unattractive home for the rich?

Then there's the decidedly mixed reception being accorded to the new album, No Line On The Horizon. Rolling Stone may have lavished it with a five-star review, the first the band has ever received from the magazine, yet there is a sense that the accolade was the equivalent of a lifetime achievement Oscar, awarded as much for past glories as for current achievements.

Among the wider critical community, an emerging consensus is that the new LP, while hardly a commercial car-crash of Pop-esque proportions (to this day, the 1997 project is the closest to a proper U2 'flop') is one of the dullest things they've ever done -- "mind numbing blandness", "staid and uninspired", "confused and confusing' are some of the pejoratives that have been tossed in its direction by reviewers.

Nor is No Line On The Horizon exactly the blockbuster it at first seemed to be. Yes, it was a worldwide number one. However, it is shaping up to be a commercial letdown in the United States, where, three months after its release, it had yet to go platinum. In its first week, No Line shifted 450,000 copies in the United States -- not bad at a time when the record industry is locked in potentially terminal tailspin. Still, this represents a sharp fall-off from the 840,000 opening-week sales of 2004's How To Dismantle An Atomic Bomb (likewise released with the record business in upheaval).

Some of the blame, undoubtedly, lies with the choice of hard-to-like Get On Your Boots as lead single -- though, surprisingly, follow-up Magnificent, with its trademark Edge shimmer, proved as much of a disappointment, stalling at number 42 in the UK charts (the worst return for a U2 single since 1987), with an underwhelming 5,157 copies bought or downloaded.

"It seems to us there could be a million reasons Horizon might not be catching fire," mused a writer at New York Magazine. "Is the dull cover not standing out enough on CD racks? Did everybody see its perfect review in Rolling Stone and assume that only their dads would like it?"

All the leaking scandals can't have helped. Late last summer, Bono put several unfinished snippets from No Line On The Horizon into the public dominion while holidaying in the South of France. Actually 'leak' is not quite right: the singer was playing rough mixes from the LP to friends when an enterprising holiday-maker realised what he was hearing and recorded snatches of the music on his mobile phone. Rather than simply sharing with friends or listening to them for his own enjoyment, he did what any modern, technologically savvy individual would: posted the entire thing on YouTube.

"In an odd way, that was pretty harmless," said band manager Paul McGuinness, a fierce opponent of internet file sharing. "A fan happened to be lurking around Bono's house. It's a public beach. Bono was playing some of the tracks as they then existed. They were pretty incomplete. By holding up his mobile phone, the Dutch fan was able to record a very low quality signal. Sure enough, the next thing it was on YouTube. It wasn't so much annoying... well, it's always annoying when people hear things before they're finished, before the band have said 'that's the way I want it to be heard'."

More seriously, the full album was accidentally leaked by their record company's Australian division a full two weeks before it was due to be released in the Northern Hemisphere. Within days, it was all over the peer-to-peer networks favoured by illicit downloaders. At least Edge saw the funny side: "The one good thing about that is a lot of our fans have already given us their thumbs up," he joked. "Even though it was fans getting it for free."

The 360° tour, too, has been a point of contention, with environmental campaigners decrying the vast carbon footprint that U2 are set to leave (some 160 trucks are required to ferry the touring party and the vast, tarantula-like Claw edifice around the globe).

"Looking at the 44 concerts, U2 will create enough carbon to fly all 90,000 people attending one of their Wembley dates (in London) to Dublin," said Helen Roberts, an environmental consultant for Carbonfootprint.com

In their defence, U2 have held their hands up and admitted that, yes, their tour has consequences for the environment. The question, says Edge, is whether you actually want U2 to continue bringing their music to the public. Would it really make any difference to the future of the planet if the Dubliners, alone among stadium rock bands, called it quits in order to salve their consciences? Fans seem to be on their side.

"The carbon footprint of this might be quite large, but the spiritual rewards to the audience of this are those that enhance a life," Mark Reed wrote in a review for the Final Word, a website popular with U2 fans. "If all life were bread and water, then there would be nothing to lift mankind above the amoeba."

As to rumours of disquiet in the camp over Bono's crusading -- well, the band have been careful to present a unified front ever since drummer Mullen told a UK magazine that he 'cringed' every time he saw the frontman palling around with world leaders, in particular Tony Blair, whom the usually quietly spoken drummer branded a "war criminal".

"I see him and Bono as pals, and I don't like that," said Mullen. "I don't think there's much of an upside to it for him, I don't think he chooses where he goes and who he meets. But as an outsider looking in, I cringe."

He was probably cringing, too, when attention started to be focused, once again, on U2's tax affairs. How, people wondered, could Bono claim to be working on behalf of the world's underprivileged when, by taking tax avoidance measures, he was depriving the Irish exchequer a huge wedge of income -- money which could have been used to build hospitals, pay for teachers, etc?

Curiously, the snark appears to have wounded Bono, who might have been expected to have cultivated a bulletproof hide in view of the ridicule heaped on him down the decades.

"We pay millions and millions of dollars in tax," he said around the time of the LP's launch. "The thing that stung us was the accusation of hypocrisy for my work as an activist. What's actually hypocritical is the idea that then you don't use a financial services centre in Holland. The real question people need to ask about Ireland's tax policy is, 'Was the nation a net gain benefactor?' and of course it was -- hugely so. So there was no hypocrisy for me."

Mullen was just as unimpressed with the criticism. In fact, he went as far as to suggest that wealthy Irish people who move part of their estate abroad are singled out for degrading treatment.

"We have experienced [a situation] where coming in and out of the country at certain times is made more difficult than it should be -- not only for us, but for a lot of wealthy people," he said. "So it wasn't personal. It was to do with the better-off being sort of humiliated."

However heartfelt, such protestations are receiving short shrift. "Bono is lobbying the Irish government to contribute to the millennium development goal when he himself avoids tax," says Marina Hyde, author of Celebrities: How Entertainers Took Over the World And Why We Need An Exit Strategy. "Where does he think the Irish exchequer gets its money? It comes from nurses and teachers and bricklayers -- workers who pay their taxes."

Still, it would be foolhardy, reckless even, to dismiss U2 as yesterday's heroes. Among fans, the emerging consensus is that No Line On The Horizon is a classical example of a 'grower' -- a record which, while initially difficult to warm to, reveals its charms with repeated listening. Critics, for their part, are unanimous that the 360° Tour is the most epic and astonishing endeavour U2 have yet undertaken, a glittering, post-modernist joyride that manages to feel both awe-invoking and weirdly intimate (the stage was designed, in the first instance, to allow U2 to get closer to fans).

Ultimately, the question is this: who would you rather rule from on high as world's biggest rock band; Larry, Bono, Edge and Adam -- or the tiresome pretenders snapping at their heels? Sooner or later, dreary bombast merchants Kings of Leon, Chris Martin and The Killers are going to reign unchallenged as the kings of rock. At which point we may well end up wondering why we didn't appreciate U2 more when they were at the height of their powers? Leaks, tax avoidance, flop singles -- all of the baggage will surely melt away this weekend when The Edge gazes out over Croke Park and plunges into one his trademark sheet-lightning riffs. Some things are just so much bigger than everyday life.

- Ed Power