(LOL @ people saying this is a rip-off of PvZ)
First and foremost: This game is amazing. I consider the setting much more appropriate (even though it lacks the name puns from PvZ), and even though the game has considerable less content/features overall than PvZ, it's still insanely fun to (re)play.
Game is also very well polished, although it may be laggy sometimes due to many enemies on screen sometimes. Thumbs up for the music - it's a great fit, and doesn't turn out to be repetitive, at least for me.
A feature I disliked at first, then found it to be OK, is the "click the books for currency". It adds some challenge and makes the game more than just building defenses. I was originally set to pull a star for this, glad I postponed this review until I changed my mind.
Difficulty wise: Learning curve is excellent - game felt extremely balanced on Normal Mode and moderately to very challeging on Nightmare mode, depending on luck due to random defenses.
Overall: A great experience affected only by some lagging now and then. This game is a must play not only for PvZ fans. It's not a rip-off, so just stop complaining and have fun.