I've been thinking about getting one myself lately, I know there's at least a handful of DS only games I'd play, it's just considering whether the enjoyment I'll get is worth the significant outlay originally ($169 for the handheld, plus $50+ for new games). Then there's the consideration of how much I'll actually get to use it.
I bought a PSP shortly after launch, and it's been a relationship with ups and downs. I've got only about 6 games for it, and although I love those games, I haven't touched it in a quite a few months, maybe even a year. Being a handheld it's not as convenient to play as a console or even a PC if you're at home (charging it, small screen, cramped fingers after long sessions, etc). If you've only got a few games for it, it's also very easy to just get bored - you beat all of them, or get to parts you're stuck on (that's a problem with any system you only just bought though). One of the reasons I'm a bit concerned about buying a DS is that I think it'll go the same way, since all the problems I've had are with handhelds in general and not the system itself. The PSP is great for mature aged games: FPS, racers, sports games, that sort of thing, its also got all the added multimedia features, but I personally don't use those much. The DS seems better for the good old action-adventure games, retro classics, and kids' games.
Then again, I might just get one, I'm in a gaming mood lately. Looking to buy a few more PSP games and get a decent library going, and I've also been considering buying an old NES, just to play all the games from my childhood again. Then again, maybe a DS/Wii would save me doing that.
Khell_Sennet, on a side note, I disagree with you on Piracy, but I'll save that for another thread to avoid taking this one off-topic again.