I bought this last weekend, and I've confirmed that the marathon is broken. I didn't even notice the option to turn on endless mode the first time, so I got quite a shock when it just suddenly ended at 200 lines. The presentation is nice, and I nearly lost when that Russian folk song I can't spell started playing at level 19 because it made me laugh. I've come to the conclusion that the game's difficulty progression is pretty borked, too, though. In the old days, before infinite spin, I could almost get to level 20 before totally losing control of the situation. In Tetris DS, the pieces start falling almost instantly at a much lower level. I have a feeling it's because they messed up the fall rate progression that infinite spin became necessary.
I'm still happy with the purchase, though. Push mode is the best multiplayer puzzle game ever. I love how you can mess with each other directly. There's nothing like removing the floor just before your opponent drops his I down a hole. Puzzle mode is good, too, but the cheery music gets old fast. I judge all puzzle game soundtracks against Tetrisphere and The New Tetris these days, so Tetris DS loses a few points there.