Turning to the topic of games people here actually play, I finished Plants vs. Zombies. There are 50 levels in the adventure mode, 9 levels of each of two puzzle modes (plus endless), 20 mini-games, and 10 survival levels (plus endless). You can also develop your own, zombie-free zen garden on the side.
Few things are terribly difficult, although I say that as someone who enjoys and is good at tower defense games. I got through adventure mode without a loss, and difficulty in other modes arises mostly when random perversity beats your margin of error (too many of X appear at once, or of X and Y together). You can almost always recover from those moments anyway, so the difficulty remains low. You might expect that from PopCap, but I found some of the Peggle levels harder than the hardest PvZ levels.
Adventure Mode is great, all the way through the final battle. And if you really like that final battle, it is also the final mini-game, so you do not need to repeat the adventure mode to get to it. Other mini-games include bowling for zombies and a PvZ Bejeweled, in which your plants are your jewels and zombies are trying to eat them while you play. Some of the mini-games might appeal to you, and I like the Vasebreaker “puzzles.” Everything is cute, of course, down to the zombie dolphins. The song at the end is no Still Alive. Steam has enabled accomplishments for Plants vs. Zombies.
If I might make three suggestions? First, I do not like the fog. It adds far more annoyance than difficulty. You can counter the fog, but why bother when you can still win easily, minus the fun of actually seeing what your plants are defeating. Second, I would like to be able to repeat Adventure Mode levels individually, the way I could with Peggle levels, or at least pick which of the five stages I play. Maybe that option exists after I beat Adventure Mode again; until then, I have Survival mode if I want to play without the fog maps. Third, I would like to be able to save my favorite plants for quick-loading. Half of my plants will be the same in each level, so let me pull those up (then click away the ones I do not want). Bonus points for letting this vary by the stage (day, pool, etc.). Extra bonus points for making it automatic: track which plants I pick, then have the most popular ones in my seed tray automatically.
Meta-game tips (since the FAQ has normal tips): once your zen garden is going, you can give the snail some chocolate and walk away (or alt-tab in windowed mode) for an hour while it collects coins. The survival levels are also good fundraisers: marigolds pump out coins while you crush the enemy. The upgrades you buy with those coins really shine in the survival modes, when you will have time to use them all.
In real life, I do no gardening at all. I am so dead.