Daniel Libeskind is excited. It is two days before the opening of his first US building, and the diminutive figure, wearing a black suit and shirt setting off cropped hair and heavy-framed glasses suggestive of a Thunderbirds villain, is telling an audience of art-world bigwigs of his love for his latest creation.  Words spin out of him, many of them with little apparent meaning. It is, he tells his eager public, "an exuberant explosion" full of "spatial fluidity", a "joyous adventure" driven by "tension and dialogue". "This building is not a static building," he declares. "It is alive."  He is standing in a grand white marquee pitched next to the building that is the object of all the fuss. And while the architect may not be the best judge of his own work, the building in question is nothing if not dramatic.  Inspired by a glimpse of the Rocky Mountains gained as he first flew into Denver, Libeskind's new building for the Denver Art Museum sits like an alien craft amid the civic grandeur of the mile-high city's downtown. Docked at an angle on a vacant plot of land, its hull shimmers in the sun, the titanium surface reflecting the colours around it, silver and ochre fading into a brackish brown. The stern of the ship is a jumble of metal boxes, stacked any which way. Its prow looms over the adjoining road, jutting out toward the museum's existing building, a grey crenellated mausoleum of a place.  That Libeskind should have completed a building at all in the US is something of a surprise. The $110m Frederic C Hamilton building has come to fruition almost by stealth, as its architect has achieved celebrity and, in a way, infamy elsewhere. The Denver project began seven years ago when the city passed a bond initiative granting $62.5m for an extension for the art museum. (The remaining $47m was raised from private donors, including Phil Anschutz, the man who owns much of Denver and a fair chunk of Los Angeles, and Bill Gates.)  Libeskind won the architectural competition to design the building in 2000, beating Thom Mayne and Arata Isozaki, even though he had just one major building to his name, the Jewish Museum in Berlin. But by 2003, Libeskind had won another competition: to design the masterplan for the World Trade Centre site. With it came much fame and much exposure. It also brought controversy, firstly over the design and latterly over the politics of getting it built. Libeskind's involvement with the redevelopment effectively ended in 2004, although he is still credited as the masterplanner.  But if the blow has affected him, he doesn't let it show - at least not in Denver. "Here," he says, "unlike any other place, optimism is not the exception but is the rule."  This sense of untrammelled optimism is personified by the building's playful sense of mischief. No line is straight, no plane vertical, no two walls perpendicular. Why have square windows when you can have triangles? Everywhere shards and fragments of the thing lurch out unexpectedly. It is an exhilarating experience. Like an inverted, extruded and exploded piece of origami, Libeskind's building threatens to swallow itself.  Which is part of the problem. For, while Libeskind has designed a striking building, a partial realisation of his forlorn and abandoned plans for the V&A "spiral", it almost tries too hard for its stated purpose: the exhibition of art. Too often with modern architecture, Libeskind tells his audience, "when you enter the building it's the same old box inside". But not here. "Inside and outside are completely in tune with each other."  The entrance to the new building is rather conventional and almost drab. A large open space with a low ceiling clad in off-white acoustic tiles, a ticket desk, sofas set at a jaunty angle, a coffee stall - it is all rather familiar. A jagged spiral staircase leads the way to the galleries, looming before the visitor like a mountain range. As you make this disorientating ascent, the white of the angled walls as deceptive as a wall of snow on a mountainside, it becomes apparent that Libeskind is paying homage to another great triumph of architecture over art: Frank Lloyd Wright's Guggenheim gallery in New York, with its graceful - and, as Libeskind notes, exhausting - circular climb to the top.  Inside, the mischief continues. Walls incline, corners invert and gallery spaces recede to sharply angled points. Occasionally, this helps the art, and occasionally, the art can help the building: a Kiki Smith statue of a guard dog held by a figure is placed at what seems to be a random position in the contemporary art gallery, until one realises that the statue is placed just so, to prevent visitors clonking their heads on the piece of masonry that spears into the room above it. Closer inspection reveals that the protruding interventions beloved of the architect have fallen foul of the custodians of public health: wooden boards run around the base of some of the walls, protecting the heads of the citizenry but playing havoc with the aesthetics.  In one or two instances the eccentricities of the building enhance the art. A series of laminated plexiglass tubes by Vasa Mihich refracts coloured light up the walls of the gallery. Likewise, an almost unseen Dan Flavin neon spills light along a wall. A sequence of burnished metal tiles by Carl André casts watery reflections on the interior of the prow at the very tip of the gallery. On an exterior terrace, a series of what appear to be air-conditioning units turn out to be a Donald Judd sculpture, lending his work an unexpected humour.  But at times the interior walls' determination to follow the contours of the exterior seem at odds with the art. One room features a selection of Betty Woodman ceramics: like leeches they cling, seemingly terrified that they might slide down to the floor below. Elsewhere, Marcel Duchamp is lost in a pointed corner. Whether this is a sly curatorial game or an unhappy consequence of the whims of the architect, it is difficult to tell.  While most modern art can cope with all this, it is the gilt-framed art from the museum's permanent collection that suffers most. Denver's extensive collection is not at its best hung in a series of galleries burdened with taupe partitions, and the overall effect is dowdy.  Early critics of the building have latched on to this disjunction. The Los Angeles Times dismissed it as "an array of the least congenial galleries for art that I've seen in 20 years". But as Dan Kohl, the museum's director of design, points out, we should leave such verdicts to the public. "The obvious question of how you would be able to hang and show art in the space will be answered by people coming and walking through the building," he says.  The city, eager to mark itself out as a cultural destination midway between the two poles of LA and New York, has embraced Libeskind. He, in turn, is threatening to turn Denver into a personal theme park. He has built an apartment building next to the museum, and he has been commissioned to create plans to redevelop a large chunk of the civic centre - plans that have so far generated more hostility than anything else. But Denver has got what it wanted: a famous architect who has delivered a unique, striking building that will attract visitors away from the coasts.
With its dramatic angles, Daniel Libeskind's new art gallery is lighting up Denver. There's just one problem: you can't hang much on those walls. By Dan Glaister.