We are looking at a building, then, which could indeed be a big good-looking shed - a building type for which the firm of Wilkinson Eyre used to be noted - but which has to be something else. The big box is broken down into four intersecting pavilions, increasing in size from one end to the other and set on a curve. It's a great image to win a competition with but, as Wilkinson admits, it made building the thing more than somewhat complicated. "Everything impacts on everything else," he remarks. "You've got to follow it through in every detail."

You come across this kind of slightly fretful design concept in all kinds of places. A provincial companion to the National Waterfront Museum, for instance, is the National Maritime Museum in Falmouth by architects Long and Kentish, which is again good in its way but comes perilously close to parody in its maritime imagery - it incorporates a kind of lighthouse look-out tower. But it also refers to wharfside warehouses, and so uses so much timber cladding that it looks from a distance like a model made of matchsticks by some bored prisoner. Oh, and note the linguistic shift apparent in the use of the word "national" in these places. It means "regional".

Back in Swansea Wilkinson also has got a bit contextual in his choice of materials. Not timber, here, but slate. While the northern side of the building is glass of various kinds, the southern side is treated to a cladding system of vertical slices of rough slate. This sop to Welshness could have turned out nasty but instead works rather well in all its variations of colour and texture and unexpected sparkle. It looks particularly good in the rain, and how many building materials can you say that of?

Few architects draw strange shapes just for their own sake - there is usually some kind of real or imagined logic driving them. In Swansea's case, the staggered form of the new building - and the design of its surrounding landscape, by the same architects - was suggested by the history of the site, which used to be a dockside railway depot with a great thick swag of railway lines draped across it. Those lines have long since gone - though the dock remains, as a marina - but they acted as a generator for the new design. The big curve also gets you round the large and hideous object of a neighbouring leisure centre, now being re-clad in an attempt to make it slightly more palatable. So the building can be seen as a series of huge wagons clanking round a bend, a memory of the industrial past enshrined in its collection. Setting it out on a curve also prevents the new building from overwhelming its other half, a restored brick-built dockside warehouse which has museum spaces on the first floor, and will have shops and restaurants on the ground level. You can walk right through both buildings to the waterside via a broad shared lobby, clasping old and new together. This is a bold and successful move.

The railway-wagon idea doesn't work quite so well, however. Wilkinson could have achieved much the same effect with a single curved building, gently rising from end to end if necessary (though it's not, really), but opted instead for the harder task of jamming his rhomboidal boxes into each other. Since the main exhibition space occupies two of these boxes but reads as a single room inside, it's a lot of effort for not very much result. Okay, so you can see the ceiling step up at one point if you look closely enough. Few will notice, or care much if they do. True, skewing the space in this way does liven things up a bit. But is it worth all the added complexity? Going wholly free-form like Frank Gehry is in some ways easier than all this fiddly slicing and jointing.

The exhibition designers are Land Design Studio, who as it happens also did the maritime museum in Falmouth. Their scheme works well, combining real artefacts with audio-visual and interactive material in a way that doesn't seem forced. This is a museum of industrial and social history, so as well as big and small bits of factory, mining and dockside kit - rather too closely packed - you have for instance an intriguing investigation into the lives and activities of people behind a 19th century census. Inevitably, much of the museum is aimed at the schools market and as usual some of the computerised stuff is too slow to respond, but it manages to be both un-nostalgic and uncondescending. The idea is that history continues: there is a whole section devoted to high-tech things happening today.

In London architectural circles, people have long talked despairingly about the Welsh Problem. How proposed good new buildings get axed and good old ones get demolished. Since devolution, there are signs that this is changing. Cardiff Bay is very far from perfect but it has the cultural content that its equivalent in London's docklands completely lacks - plus the seat of government in the form of Richard Rogers' nearly-complete Welsh Assembly. Caernarfon has its new arts centre by Richard Murphy, and now Wilkinson has now given Swansea what is effectively the combined museum of industrial and maritime Wales.

None of these architects is Welsh: the burden of nationhood falls on the shoulders of Jonathan Adams, designer of the stealth opera house known as the Wales Millennium Centre in Cardiff Bay, which is better than most people give it credit for but cruelly doomed always to be compared with its aborted predecessor by Zaha Hadid. Whatever, it is time to stop hand-wringing about new public architecture in Wales. The soi-disant National Waterfront Museum may be trying a bit too hard for its own good, but it is trying, and it is built. Now perhaps it is time to consider a return to the Big Shed.

Wilkinson Eyre architects, double Stirling Prize winners:: www.wilkinsoneyre.com
Swansea's National Waterfront Museum: www.nmgw.ac.uk/nwms/