It's very funny at times. My problem is that I don't find spoofs particularly funny. There's just too much action, and too much effort put into creating Apocalypse Now-type scenes, for the comedy to hold its own. As others have said it's a very well-written film and Stiller's direction is fine, but the humour is swamped by the special effects and over-researched music.
But well worth seeing, for Cruise (obviously) and for the quite vicious swipes at Hollywood stereotypes in the opening sequence.