This is not a "great" film, but it's elegant, well-shot, and staffed with superb actors & actresses that know their work & do it well.
You have all read the plot line, so I won't dwell on that. I will say, though, that viewers searching for a typical "story-conflict-wrapup" will be disappointed. This film is about life...several lives... and we are shown a brief window into those lives.
Lucy's story (Lucy = Liv Tyler) is, I believe, the least interesting -- we always observe her, and never get into her head, and yet know what she's about. But...she's 19... she knows very little... and Bertolucci knows that.
The real stories are how the others, older, react to her and to each other... lust, memory, envy, nostalgia for lost youth, jealousy, pride, recognition, understanding of the motion of life...
All of these evoke other stories that, unfortunately for all of you that want a nicely wrapped-up movie, you are going to have to make up in your own heads. But that's the beauty of this film.
While I hate to generalize, teens will dislike this movie, as will adults who think that suburban life is pretty good. It's never explained what any of these people actually DO, and I know that's an important problem for many movie goers.
But the rest of you... give it a try.