We've played Splinter Cell on the XBox and PS2 and loved it. Even though the GBA version is far simpler, it's still quite fun!
Once you get past that hurdle and accept this as a "fun stealthy game", it really is quite enjoyable. There are the same general mission aims as in the other versions, so you have hostages to rescue, contacts to talk with and enemies to kill. You have to use stealth as much as possible, shooting only when necessary.
The graphics are reasonably good for the GBA system, and the music works quite well. The controls are extremely simple to master, so there isn't a lot of button mashing. Most areas are more like puzzles, trying to figure out exactly what the best way to get past an obstacle is.
Highly recommended for spy and stealth game lovers.
Buy Splinter Cell GBA from Amazon.com