It's not an architectural novelty, you are correct Joe: Hundertwasser designed "his" houses all similar.
But - besides that spa mentioned above - it's the largest assembly buildings he planned:
(Source: Microsoft Virtual Earth)
Clean that up and plant some fresh grass then add some interesting public art and you have yourself a public garden on the building's roof. Or a miniature golf course.
I would love to live in an apartment building like this one. Great sense of architectural whimsy.