I got a DS Lite two days ago and I am definately having fun. The battery life is great- the only time I have had to charge it so far is tonight, using it several hours each day, second darkest setting. It feels durable, sturdy, and is a bit heavier than I imagined. It easily slips into my pocket. The sound quality is quite bad, though what do you expect. The only other downside I have noticed is that GBA paks protrude slightly when inserted. It still fits in my pocket, but not in the protective case for the DS Lite I got. Games I have are New Super Mario Bros, Sonic Rush, Electroplankton and Final Fantasy IV Advance (GBA).
Mario is really a great game. The main game provides a great amount of fun (and some nostalgia) with a few difficult parts, though I have not gotten stuck for too long. Some of the minigames that come with it are quite addictive. There is even a simple version of video poker and some other card games which remember your progress when you quit to resume later.
Sonic Rush is more tough to pick up on, and once you do it is a lot of fun- there is a good amount of variety to the game- very interesting, fast 2D levels, 3D boss battles and 3D special stages that use the touch screen- these are especially fun, and difficult.
I find Electroplankton interesting but I can't say it is for everyone. It is very open-ended, there are no objectives, you just have to try to make cool sounds using the abilities of the various plankton. It can be very rewarding when you find something that sounds good, and it becomes easier with experience. It becomes apparent playing this that the mic on the DS is not of good quality and is not very sensitive. It will only pick up your voice if you are within about a foot of the mic (though this can be good if you're in a loud place), and it sounds sort of like it is coming through a phone line- probably sampled at low rate.
Can anyone recommend any good, long RPGs for DS or GBA?