While the WiiWare version is vastly superior, the fact that you can take this one on the go makes it worth the purchase
Gameplay: If you've never played Dr. Mario before, first of all, what's it like living with your head in the sand? Second of all, here's what you need to know: Dr. Mario has been around since the NES days, and is one of the most beloved Tetris-esque games around. Instead of clearing lines, however, you'll be killing viruses. Dr Mario will stand off to the side and throw multi-colored pills. Your job is to match the color of the pill with the color of the virus you see on screen. This is not nearly as easy as it sounds. Each pill has two sides (usually each with a different color) and there could be anywhere from 3-100 germs on screen. It's crazy hard. But it's also crazy fun. That was not a great explination, but all I can say is that if you like Tetris, you're going to love Dr. Mario.
Complaints: The classic Dr. Mario gameplay is still there. The problem is what ISN'T here. Dr Mario Online for WiiWare has up to four person multiplayer, online multiplayer and leaderboards, and an all new "virus-buster" mode (which would've been perfect with the DS's touch screen). You'll get none of those extra features with Dr. Mario Express
Bottom Line: If you're looking for the best Dr. Mario experience out there, go with the WiiWare version: Dr. Mario Online Rx. But if you're just looking for an addicting app for your DSi, Dr. Mario Express comes highly reccomended. The truth is, Dr. Mario, like Tetris, just makes more sense on a handheld. It's the kind of game you take with you on the go.