Only in Paris. Only in Paris would they queue five abreast for half an hour on a warm, sunny afternoon for the privilege of peering at an exhibition about 18th-century military architecture. Only in Paris would they barge their way to the front to furrow their brow at a display on 19th-century architectural conservation.

And only in Paris would they prefer to analyse gargoyle design rather than, I don't know, watch telly, go shopping or engage in any other sensibly unintellectual Anglo-Saxon pastime.

In September President Sarkozy, flanked by 14 of the world's top architects, opened what he billed as the world's largest architectural museum, the Cité de l'Architecture et du Patrimoine. It was the first cultural salvo in the national metamorphosis that won him the election, beginning, he pledged, with France's architectural renaissance after 20 years slumbering in the shadows of Mitterrand's grands projets. Culture, he proposed, must be made more accessible to the masses.

From the look of the hordes at the Cité (your usual high-minded, museum-going, middle-aged Parisian bourgeoisie in tortoiseshell specs and Maigret raincoats), the masses sure ain't here swotting up on Le Corbusier. But no country needs lessons on going to museums less than the French. The Cité is bustling - it's the capital's latest must-see cultural hotspot.

As its name suggests, this is most definitely a city. It sprawls over 23,000sqm (247,600 sqft) through an entire wing of the gargantuan Palais de Chaillot, a 1930s Art Deco, vaguely fascist pudding that faces the Eiffel Tower across the Seine. It's vast. You could get lost in it.

The much-delayed Cité has been more than a decade in the making. Plans for a national heritage centre were drawn up for the Chaillot site in the 1990s, though they were put on hold after a fire in July 1997. They refocused soon afterwards to become today's Janus-faced behemoth, fusing the Institut Français d'Architecture, France's contemporary architecture gallery, the École de Chaillot conservation school and the august Musée des Monuments.

But a national museum on architecture has been nearly two centuries in the making. The first moves were made after the French Revolution, when the destruction of monuments stirred pioneers of conservation to document France's architecture as a vital tool in reformulating national identity. But it took France's answer to William Morris, Eugène Viollet-le-Duc, to found the Musée de la Sculpture Comparée, opening on the site of the Palais in 1882 with a remit to educate the French public about their national heritage. It produced hundreds of one-to-one scale casts of chunks from France's architectural greatest hits.

It is these huge, magnificent casts - statues, sculpture, tombs, portals, columns, frescoes, angels, Last Judgements, beasts, devils, entire chunks of Reims cathedral that have swollen to a collection of several thousand after that museum became the Musée National des Monuments Français in 1937 - that form the core of the Cité, occupying the huge ground-floor galleries.

It may be the biggest architectural museum in the world, but, alas, it certainly isn't the best. In part this is the fault of the core collection, a highly partisan affair, dominated by French architecture. This is its strength and weakness. France is well represented by architecture in almost every century since the Middle Ages, a spread and evenness unequalled in Europe, so any history of architecture has plenty of local talent on which to draw.

However, displayed here it does give the impression that architecture of any merit ceases to exist beyond the border. And, while there's no denying the impact of the mammoth casts and models, in any well-balanced architectural museum, such copies can be only a support act, not the main event. The Cité's collection pales beside the Royal Institute of British Architects' million-plus original drawings and models at its comparative tiddler of a gallery at the Victoria and Albert Museum, or the stunning Modernist collection at the Canadian Centre for Architecture in Montreal. Here there's size, but little to fill it.

In the main, though, the fault lies with the curating, or desperate lack of it. The highlight is the first-floor 19th and 20th-century gallery, where at least there's some discernible story being told. Here, the centuries split into two broad themes: Designing and Building, and Architecture and Society, each then subdivided into a series of often fascinating mini-essays.

There are some real treasures, such as a beautiful, modern model of the Crystal Palace being built; a full-sized fragment of the Perret brothers' concrete Église Notre Dame de la Consolation, in the northern Paris suburb of Le Raincy; a model of France's socialist answer to England's garden cities, Tony Garnier's Cité Industrielle (1899); and a full-scale reconstruction of an apartment from Le Corbusier's Unité d'habitation in Marseilles, in which you can judge for yourself this experiment in communal living in the sky.

Downstairs, though, in the vast main galleries, the Cité is less nippy populist than decrepit history master, using Viollet-le-Duc's collection of casts as an excuse to tell a monotonous story of architecture from the 11th to the 18th centuries. It is comprehensive, if dusty dry, on the emergence of Gothic from the Romanesque, but patchy post-16th century, where the collection is weaker. The tale of aristocratic patronage, challenging the Church's Gothic monuments with the glorious Renaissance and Neo-Classical chateaux, is skipped through. There is little context and antiquated architectural history, with styles seeming to develop magically, unconnected to people, politics and society.

This is a terrible shame. The collection's paucity notwithstanding, a spirited story telling the tale of why murals, statuary and stained glass turned buildings into giant picture books in the Middle Ages, or why Gothic emerged in France at all, is vital for fulfilling Sarkozy's aim - extending the reach of the museum beyond the usual middle-aged, middle-class audience.

The overall problem, though, is the fragmentation of the Cité. There is no curatorial harmony between the galleries, or between the three institutions, so the tone nips awkwardly between elderly history teacher to avant-garde installation artist. You will find occasional gems, such as a sweet temporary exhibition on architectural toys, Qu'est-ce que tu fabriques?, with Gaudí's giant Sagrada Familia church made from Meccano - but no consistency.

Ironically, this problem is worsened by the Cité's biggest problem: its architecture. While the restoration of the Palais is neatly done, its conversion is a mess. The museum is appallingly designed - unforgivably so. Admittedly, it would take quite a master of space to weave together the disparate organisations in this sprawling monster. But the convoluted circulation, the Kafkaesque corridors, the shoddy detailing and the eccentric plan, banishing the three institutions to the corners rather than properly integrating them, and consigning the temporary exhibition spaces to the basement, make a mockery of what's meant to be the national showcase for design.

At the opening, Sarkozy called the new museum a symbol of France's urgent national reformulation after years of crisis, the embodiment of “our entire country, the territory of our values, our references, our hopes”. Heaven help France.

The Cite de l'Architecture et du Patrimoine, www.citechaillot.fr

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