Many other might-have been buildings hover like vengeful ghosts over the new Sainsbury Wing. Some of the earlier proposals in the 50-year history of the site would certainly have looked better from the outside. We don't know how well they would have turned out inside. And this should be the crux of any debate on the building.

Outside, the Venturis' building is not terribly good. Nor is it terribly bad. It's just OK, given that Trafalgar Square is surrounded by other buildings that are average-to-bad (the shining exception being St Martin-in-the-Fields). It will swiftly become virtually invisible to the public, though the sharp-eyed may notice Venturi's touches of Pop, such as gaudy little Egyptian columns. But step inside, and marvel.

The rooms on the top floor house the National Gallery's early Renaissance collection. Venturi and Scott Brown have created galleries with something of the religious atmosphere of a church, without any ecclesiastical imagery apart from the round-arched apertures between rooms. This is a series of lofty, seemingly old-fashioned lantern-roofed galleries, owing something to Sir John Soane's tiny early 19th-century Dulwich Picture Gallery. But daylight is carefully controlled by filtering devices, and the ceilings (acoustically dampened to cut down echoing footsteps from the wooden floors) conceal a mass of air-conditioning equipment. Very nearly state-of-the-art, but by no means shouting the fact.

The sequence of spaces is finished in good hard-wearing materials, with a very muted colour scheme. One direct vista with a false perspective across the rooms, defined by fat recessed columns, greets you as you enter from the top of the grand staircase. Otherwise, the interconnecting doors are arranged to give informal, even sudden, views of the paintings as you wander about. There is no craning of the neck at any architectural jeu d'esprit. People will come here for the collection, not for the architecture.

Elsewhere in this surprisingly big building you will find a brasserie overlooking Trafalgar Square, a lecture theatre with a horrible Venturi-designed carpet, and, downstairs, a set of artificially lit galleries for temporary exhibitions: satisfyingly tall rooms for what is virtually a basement. If you really want to find architectural witticisms, they are there to be found. Like the gallery wall that swerves suddenly to avoid crashing into he middle of a window. Or like the ceiling over the main staircase. Presumably to add grandeur, the Venturis have incorporated curved steel roof-trusses, vaguely reminiscent of a Victorian railway station. These have no function other than ornament: to demonstrate this, they are hung from the ceiling with no visible means of support. One should deplore such incorrectness, but I feel strangely indulgent towards this building, and to the Venturis. It must be the ghost of Lutyens, giggling somewhere.