The first few levels are quite easy. In order to amass your green army, you've got to collect sunlight. Some plants, like sunflowers and certain mushrooms, give off sunlight that you can gather. The rest you collect from the sky (in the daytime, at least). As you complete each level, you're offered another plant.
Many little touches make this game great. Select the help menu and you'll find a "hand written" note that says, "When the zombies show up, just sit there and don't do anything. You win the game when the zombies get in your houze." One zombie variation gets especially angry when you knock his newspaper from his hand while another, dressed in a red leather jacket, white socks and black shoes, summons dancing minions to launch a well choreographed group assault.
Zombies drop coins that you can exchange for additional features, like the number of plants available. Asset management gets harder in later levels as you can only choose a few plants for each assault. Just remember, you always need sun!
Beyond the game itself (there are 50 levels in all) are several mini games, puzzles, survival mode and even a peaceful zen garden. Additionally, you can browse the almanac to see exactly which types of zombies and plants you've encountered as well as what's left. The music and sound effects are fun. I found that it looked jaggy in full screen mode, so keep it in a small window.
Plants Vs. Zombies isn't Eve Online, and it's not meant to be. As a fun distraction during break time or leisure time, it's a great little game. You can play for 60 minutes for free. After that, you'll have to pay $19.95US. It's definitely worth it. If you just can't get enough, check out the extras.