I just got a ds with one of these
http://www.dealextreme.com/details.dx/sku.1202
I have 45 games on my ds right now!
I highly recommend it.
Personally I found tetris ds a bit disapointing in some ways. On the original tetris I could barley make it past level nine. but in this one i can make it to like 17, and it stops at 20! You can keep spinning the pieces forever and they dont stick until you stop. My friend who is a tetris wiz, has no where else to go with the regular single player game.
I must admit though tetris in any form is fun, the music and graphics are good(although i miss the classic music too!) and the online play sounds relly good even though i havent tried it.
Ive only had a ds for a few days so ive got alot of games to check out.