I remember having seen Plants vs. Zombies all over the place a couple of years ago when it was really popular, but I never got around to playing it. I don't know why, but I always had it in my head that it was a game from Newgrounds, not Pop Cap. I tend to shy away from Pop Cap games because they are always trying to get you to buy full versions of their games. You all played the game, and you all had to sit and watch a short commercial between levels. I can stand an ad spot before the game launches, but splicing commercials in between levels on a trial version of the game is a bit devious - either give us the full game for free with ads, or try to hook us with the trial, but not both.
Anyway, I completely broke the game. What worked for me was putting down as many sun-shrooms as possible. When the first zombie came, I would plant a squash instead of a pea-shooter. With the 50 points that saved me, I would be able to keep spamming shrooms until I had a strong economy and could pump out repeaters as fast as the game would allow me. I think playing StarCraft2 got me into the right state of mind for most strategy games - rush economic development.
After beating the trial version, I tried the survival mode which pits endless waves of zombies against you. There was also a really interesting game mode included with the trial where you have to break a series of vases on your lawn, and either a random item would pop out for you to place or a zombie would pop out and scurry for your front door, who you would have to stop using the aforementioned items.