I'd say get one. I talked my girlfriend into buying a DS Lite last year, and we've played quite some games:
- NSMB - just great
- New Princess Peach - too easy, but it's my girlfriends favourite game (yeah, I know, sounds cliché, but she says so)
- Yoshi's Island DS - good game, but not as inspired or balanced as the original by far
- Phoenix Wright 1 & 2 are cool games if you like the adventure style, but I personaly found the dialogue and protagonists a bit too weird... probably "lost in translation"
- Mario Kart DS - one of the best MK games, great controls and tracks
- Mario vs. Donkey Kong - cool and very original puzzle game
- Another Code / Hotel Dusk - I didn't play them, but my girlfriend did and enjoyed them a lot. AC is way too short, and both games are VERY text heavy
And that's just a tiny fraction of the games available. Although (as you live in Europe) there's not hardly as much choice here as you have in the US or even Japan.
Currently playing: NSMB (Wii)
