Of course, you're probably not expecting great sound from the GBA version and you'd be right to think that. There are a few sound samples that just get used over and over again. Nothing exciting, but better than what handheld users tend to be used to.
The graphics of the GBA version are good and thankfully chunk-free (unlike some of the Atari 2600 look-alike games that we see on the GBA). However, purists of the SMB (no, not Super Mario Bros.), will be quick to point out that there are no cut-scenes detailing Dr. Badboon's obsessive-compulsive, megalomaniacal crush on Mee-Mee...
The mini-games are excellent fun too. One personal favorite is Monkey Duel, where you link up with a friend and, well, duel with monkeys. Of course, Monkey Bowling also offers countless hours of fun. Just don't think your bowling score will be automatically raised.
No matter how you look at this game, there's one thing that rings out most prominently. The fact that SMB is finally portable means you can take it to the can with you and let your legs and ass go numb. Not that I'm advocating that you do this or that I've done it. Just a random thought.