Plants vs Zombies for the iPhone is a fantastic new game by Pop Caps. I have to say that I've spent hours playing the game... which makes it a great value for the money.
Though the game starts off easy, it slowly becomes more difficult, and soon a strategy is needed to survive the levels. Casual Gameplay has put together a very nice walkthrough article to assist folks with forming a strategy.
If you have an iPhone and want a low cost game with good graphics and great playability, then download Plants vs Zombies. If you need a strategy to win, check out Casual Gameplay.