The dpad is typical nintendo shit and the system is just an ergonomic nightmare. Be prepared for sore thumbs and a sleepy right arm unless you intend to play on the bed, laying on your stomach and supporting the DS and your hands with a pillow.
A lot of games make poor use of the stylus as well (either by forcing the player to use it in a game where it shouldn't be used or by awkwardly using it).
Still, there's a lot of great games on the platform and it's where most of the quirky, niche stuff is going to gravitate towards (since they're more likely to find an audience on a platform that's sold ~40+ million units so far). I don't really recommend playing GBA stuff on it; stick to emulators for that. Why play on a <3" screen when you can play on a >17" screen?
SNES and Genesis emulators on the DS still suck. There's some snazzy, interesting homebrew, but 16 bit emulation isn't among it.