Plants vs. Zombies is a great and diffiult game with tons of variety.
The learning curve isn't bad really. Its pretty simple from the get go, but as things go on, it becomes increasingly difficult as it becomes harder to decide which plants should be brought along and which ones you are better off without. Increasingly resilient zombies make things more difficult as well. And all of it is well done and balanced, but if you bring the wrong plants into a level, you might as well start over, because you most likely won't make it to the next round.
There is so much variety in Plants vs Zombies, there are tons of different plants for you to grow to aid you in battle, and plenty of different kinds of distinct enemies that require different tactics to defeat. There are also minigames you can play that are actually quite great as well.
The game plays best on PC. I have only played the PS3 and the PC version, but the interface and controls for the PC version are flawless, and don't seem to be able to withstand improvement. The game looks great on all platforms, clear, colorful, and beautiful.
Plants vs. Zombies is a great game to pick up. You will definitely enjoy it, even if you begin to get frustrated at the later levels. If this happens, you can always take a break and play a few mini games and then try again later.