I love puzzle games. I own Tetris and Mr Driller for the DS and I think they're great, but I bought Zoo Keeper for half the price of either of these and it blows the competition away. For one thing you can actually play ZK with the stylus. Tetris and Mr. Driller play like slightly upgraded versions of their GBA versions. Good but hardly original. Zoo Keeper, on the other hand, is based on the 'Bejewelled' PC game which is designed to be played with the mouse, a puzzle game begging for the DS treatment.
More addictive than Polarium and Meteos combined, simpler to learn than Tetris, cuter graphics than Mr. Driller. In short, it's a brilliant game. There aren't too many playing options but the main modes are so awesome that that's not a problem. I've played the two-player version with my girlfriend (it's a one-cart 2 player mode, by the way) and it's lots of fun too.
In short, if you want DS's version of Tetris, *don't* buy Tetris DS, save yourself some money and buy this little gem.