I really enjoyed this -- I'm a big fan of movies that mess with your mind and leave you with a lot of questions and ideas to debate, and this was a stellar example. But then, Terry Gilliam is always good at that (well, almost always. Let's just forget about Jabberwocky and The Brothers Grimm, shall we?).
I particularly liked the way it handled the time travel theme and the avoidance of paradoxes -- the way events in the past and future intertwined and fed into each other.
It was also really well done aesthetically -- the art direction was really great, and I wish I'd been able to see it on the big screen. The future scenes had a similar feel to Brazil in a lot of ways, and even the present scenes were often really visually compelling.
But perhaps the most striking thing about it was that it featured two actors I normally don't much like, Bruce Willis and Brad Pitt, and they both delivered amazing performances here. Pitt especially -- I'd seen one or two films before that made me realize he could in fact actually act (contrary to what I'd originally thought), but this one really outdid them. I actually found myself asking my friends at one point "Are you SURE that's Brad Pitt?" This is probably the most memorable performance of his career (though admittedly that may not be saying too much).