Two reviews that I strongly endorse:
Those are no small themes, and "Waking Life" - fun as it is - is no small movie. It's rare, in fact, to see any film tackle such obviously weighty subjects in so thoroughly amusing a fashion. Even when the talkers are pretentious, they're engagingly or even hilariously pretentious. These are the kind of intense, half-playful confabs about the meaning of it all that many of us remember from college. If you remember them fondly, you should be delighted by most of "Waking Life." If you dislike talk or movies that touch on the abstract, you probably won't. One of the great things about "Waking Life" is how completely it overturns many ideas about what a film - or even an animated film - should be, while at the same time it unfolds with what Andrei Konchalovsky calls the "logical unpredictable." You can never guess what's going to happen next - at one point, we're in prison listening to an inmate's psychotic tirade, at another we're cruising the streets with political dissident and his bullhorn - or where Wiley will wake up next. But it always seems right.
I loved every moment of "Waking Life," but I know it's no use trying to push it on everybody. The film is truly special, truly different - a wondrous talky roundelay about and for people who love life. Watching it feels like a dream that never ends, and, in this case, you don't especially want it to. (Michael Wilmington)
and, almost of course, Roger Ebert's: I find that I want to quote his whole review here, so I really encourage you to follow the link.