The graphics in Shrek the Third are pretty good for a GBA game. Of course, you're not going to get movie quality animation, but I found the characters to be a fair representation of the animated characters. For each different land that you go to, you get new enemies and landscapes that are quite different from the previous location, so you don't feel like you're just playing the same board over and over again.
The music and sounds in Shrek the Third are quite enjoyable. The music is pleasant to listen to, but doesn't distract you from the game. The characters each have unique cries of pain when you let them get hurt, and the bad guys also have different exclamations when you injure or kill them.