If you like Jim Carrey and his facial expressions, you will definetly like this movie. I know it's not for everyone and I know a few people who don't like this movie simply because they don't like Carrey.
I've seen it probably 30 times and it never fails to amuse me. My favorite parts have to be the scene where Ace meets Lt. Einhorn at Emilio's desk. "I came here to confess, I was the second gunman on the grassy knoll" ... "That's what turns me on about you, your attention to detail ..." and when he meets Ray Finkle's parents.
I also think the plot is good, at least I didn't figure it out when I saw the movie for the first time!