It's laugh-out-loud funny, but best seen with people who have the same sense of humor as you. Crude and childish in parts but in a good way. Robert Downey Jr. and Tom Cruise are awesome.
Mr. Mouse is not quite as verbiose as I am. But in a good way.
Edit 9/12/08. Finally saw this movie and I must say that for once I completely agree with Mr. Mouse's assessment: extremely funny and crude, and RDJ and Tom Cruise were definitely awesome. (I can't think of the last time I thought TC was great in something ... Born on the Fourth of July, maybe?) A better student of film than I could enumerate the iconic movie shots that Ben Stiller paid homage to/mocked ruthlessly in TT - it was all very excellent and, I think, the best movie Stiller has ever made.