America’s greatest (and most hyperbolic) architect turned out, in a way, to be right. The Guggenheim, which finally opened in 1959, is indestructible as an idea, if not as concrete and glass. With its creamy, cantilevered curves, and its soaring interior space and vertigo-inducing overhangs, the spiraling Guggenheim embodied the futuristic spirit of the atomic age. Hovering like an elegant UFO among the upright dowager apartment houses of Fifth Avenue, it is Wright’s most famous creation and probably the most unforgettable building in America.

So when the museum’s top brass decided in 1985 to add on to this icon, architecture preservationists and the building’s neighbors went nuts. They fought the addition, designed by Charles Gwathmey and Robert Siegel, stalling construction for years, and prompting some taming of the design. The museum that critics first decried as looking like a washing machine or an inverted cupcake was now so beloved that changing it inspired yet another metaphor. People said the boxy, 10-story annex, wedged behind the Wright rotunda, would make the whole thing look like a giant toilet.

Next week, two years after it closed for construction, the Guggenheim will reopen, just in time to commemorate the 125th anniversary of Wright’s birth. The museum is tossing a series of black-tie parties to celebrate its new incarnation, but all the champagne in New York won’t douse the controversy over tampering with a masterpiece. It’s a perpetual, thorny question: should a building be so sacred for its artistry that no matter how its it must never be altered?

To many critics, adding to the Guggenheim mirrored the bigger-is-better policies of its director, Thomas Krens (page 62)-though the idea for the annex was hatched under his predecessor, Thomas Messer. In truth, the Wright building always had deficiencies as a museum. Even before it opened, a group of abstract expressionists, including Willem de Kooming and Robert Motherwell, protested that it would be a lousy place to see their art. They were right. Anyone who wants to get some distance on a big, splashy painting hung in one of the mean, low-ceilinged alcoves must back right into the railing along the spiraling ramp. Critic Ada Louise Huxtable, reviewing the Guggenheim in 1959, called it “less a museum than it is a monument to Mr. Wright.” The airy new Gwathmey-Siegel galleries in the annex add 10,290 square feet of space that will be hospitable to all kinds of art. And new office space in the annex means the Guggenheim could move staff out of the old building’s small rotunda and restore it as a wonderful new space.

What’s undeniably successful about the new Guggenheim is, in fact, the interior restoration of the Wright building. It is dazzling. Wright died, at 91, six months before the Guggenheim opened; from the start, the museum’s director, James Johnson Sweeney, subverted some of the architect’s intentions. The top turn of Wright’s spectacular spiral was walled off to make room for a conservation lab and art storage. And the skylights were covered up to protect the paintings from the sun even though Wright had wanted art to be seen in “naturally changing light. " Now the public can take the ramp to the very top of the rotunda. The skylights, with protective glass, are uncovered, flooding the space with daylight that plays up Wright’s fabulous sculptural curves. And people can wander into the small rotunda, a magical space of more curves and light, with Wright’s central stairwell shaped like the prow of a ship. From there a visitor can see into the big rotunda and the new galleries in the annex. “We wanted the building to reveal itself on itself,” says Gwathmey. “You could look back on it in many different ways.”

The gracious new galleries mesh with the old unobtrusively: you enter the annex from various points off the ramp, all located discreetly behind the elevator shaft. It’s a Wright-like idea to go through a compressed passage, then come into an explosion of light and expansive space. The new galleries are full of sensitive details and some wonderful moments: the cast-concrete cornice of the small rotunda, for instance, curves into one gallery by piercing through an exterior glass wall.

Wright believed that architecture should be linked to nature, and the new rooftop terraces–one atop the small rotunda, the other circling the big copper-clad dome-put you up near the leafy treetops of Central Park across Fifth Avenue. “Pretty hot, huh?” asks Gwathmey, giving a tour on a sensationally sunny day. Not so hot is the close-up view you get of the new Guggenheim’s most awkward flaw: the way the sensuous curve of the big rotunda crashes into the facade of the new annex. Gwathmey-Siegel’s addition is meant to be a clean, modernist backdrop to Wright’s dramatically organic icon. But it’s not all that quiet. In its crisp details, with a plaid pattern inscribed on limestone (the old Guggenheim is covered in Gunite, the plastery stuff swimming pools are made of), the annex distinguishes itself from the Wright building-which is good. It also stands in strong counterpoint to it-which isn’t. The preppy meets the bohemian, the rational meets the sensual, and the marriage isn’t quite made in Heaven.

In defense of the annex, the museum staff and Gwathmey point to Wright. In 1949, Wright made drawings that show an addition of similar design and dimensions. Wright wanted to mask an apartment building that bordered the site, says Bruce Brooks Pfeiffer, archivist of the Frank Lloyd Wright Foundation and editor of Wright’s collected writings. But Wright’s plan called for a tower only 25 feet wide (the new annex is 35 feet wide), so it wouldn’t have intersected with the big rotunda. Still, Pfeiffer believes the Gwathmey-Siegel annex “fills Wright’s intention.”

Clearly, the Guggenheim intends to be a serious patron of modern architecture. Besides commissioning Gwathmey-Siegel, the museum asked Japanese architect Arata Isozaki to create a Guggenheim branch in an 1881 building in the Soho district of New York. Isozaki’s design is a masterpiece of simplicity that lets the loftlike spaces and rhythm of the original cast-iron columns speak for themselves. The Soho branch will open next week, too.

The museum will continue to take heat for adding on to a Frank Lloyd Wright monument. Is that fair? Not really. As great as the Guggenheim is, it’s a building in a city, surrounded by high-rises. The annex fits into the neighborhood and is on the least obtrusive spot on the site. Regrettable as it is that the swoop of the rotunda hits the new backdrop, the Wright interior isn’t compromised. The museum is richly enhanced-both in its restoration and in the new ways it can serve the art and the museumgoer. Call it a fine comeback. Or better yet, a bounce-back.