The World Ends with You is definitely worth a purchase if you haven't got it already (it's £15 on play, so there's no excuse not to) One of the best DS games available, and a stunning game.
Phoenix Wright 3 is out here soon (or it might be out already, I'm not sure), and Apollo Justice is available now. They're both definitely worth getting. If you've never played a PW game though, track down the original first, as the whole series is great.
I'd also recommend importing Professor Layton and the Ouendan games in particular if you haven't already (but
if you haven't played Ouendan/EBA before)
Trauma Centre 2 is also great, especially if you've played the first. Hugely enjoyable, though possibly a little too similar to New Blood on the wii. Ninja Gaiden is a short but sweet addition to the DS library.
There are many gems that I haven't got around to buying yet, either, such as Viva Pinata, Space Invaders Extreme, BangaiO, Soul Bubbles Final Fantasy 4 and Final Fantasy Tictacs.
Fire Emblem DS will be out before too long too, with online too, so that will be a great addition to everyone's DS collection. There's Chrono Trigger and Castlevania in the future, too.
After reading ign's review, I'm now looking forward to
Lock's Quest, tooThere is a lot of shovelware for the DS, but there are a lot of great games too which tend to get overlooked by some people. Admittedly, there is a better selection of games at the moment if you import, though.
I'm bound to have missed off a lot of stuff, too.