They're pretty cool. The biggest problem I have with them is the screen is really dark, it isn't at all backlit. So, you have to use it in good lighting. They make lights for them which are ok, not great.
The screens are pretty big and colorful for such a small unit. My kids each have one, and the array of games for them is pretty extensive (you can use any GB-Color game as well as the GBA games and the E-card reader as mentioned earlier).
Also, I have heard (though I haven't researched it) that you can get an adapter to play any of the GBA (and presumably GBC) games on the Gamecube.
Get some sort of Rechargable battery source. I use Ni-mh rechargeables to good effect, they work very well since they are just AA batteries and can be used in many things, generally you can get a charger with 2-4 batteries for around $10-15. Those work for quite a while and are quick to recharge (I have one which charges up 4 in about 4 hours).