I've seen it and I loved it. I raised it on a thread about About Schmidt (which was from the same writer/director) and was viciously attacked for liking it! I think it's a classic - a really funny movie.
Sideways is quite a slow film but the comedy is played out quite well and Mr Giamatti really excels in it. The ending is a little bit of a disappointment but in the spirit of the film it makes perfect sense. I have to say I liked it a lot.
For some reason I always pair it with Garden State. Maybe because the same person recommended both films to me. Or maybe because I was squiffy when I watched both of them. I think if you like Sideways you'd probably enjoy Garden State as well though.
I don't think there's anything "classic" about the film. It's a fun, sweet movie with a strong cast. I enjoyed it very much, but it was over-hyped at the time far too much.
one of my all time most hated movies, maybe I wouldn't be so scathing about it if A, it hadn't been hyped to the hilt as another 'greatest film ever made' and B, if it hadn't have been directed by the same bloke that made Election (one of my favourite films), what the hell happened?