Yes, you should get a DSTWO. It's currently the best DS-flashcart out there right now, and the most recent 3DS/DSi update still runs the flashcart... if you update it manually via another DS (Like the phat, yes. It's a pretty tricky process if you don't know what you're doing, but this forum and the DSTWO forum can help you with that.)
The DS games are emulated almost perfectly, with the occasional bug that gets fixed within an update or two. The GBA games never run at a perfect framerate if you have sound enabled, but it's usually close enough for the average gamer (specifically, frameskip is always at 2). The SNES emulator is far from perfect, but it can run some games SNemulDS normally can't. Keep in mind that all of these things can be fixed with a simple update, whenever that happens.