This game is the bomb. It's a little bit 'tower defense' I guess, if you need to classify it, but it's just so satisfying. It starts out easy enough that I actually managed to suck a non-gaming friend into completing the whole game on iPhone, but is hard enough that I was never bored and failed a few times here and there. I liked acquiring new plants to fight with, having new zombies to fight, having different environments to fight in, completing Adventure Mode and accessing Free Play mode and the a.w.e.s.o.m.e. Mini games. Even playing Adventure Mode through a second time is set up a bit differently, which keeps it interesting.
Also, the descriptions of the plants and zombies in the Almanac are pretty funny. As are the notes the zombies send from time to time.
Honestly, I could bang on about this game for ages, but perhaps the best evidnece of PvZ being a great (and highly replayable) game is that out of all my consoles and gadgets, if I have a spare moment where I want to be occupied, this is the game I'll play. If you have an iDevice, and are in the market for a cool game, Plants versus Zombies is worth some serious consideration.
Speaking of who would win in a fight, my sister and I have been contemplating Bear Grylls vs Chuck Norris. I say it's indubitably Bear, maybe one day I'll tell you why.