Hum... Is it really good? I've often liked tower defense games but they often get repetitive and boring quickly...
Depends how you define repetitive. It's one of my favorite genres and this is one of the better games in it. If you don't like this one, there isn't much else to recommend that's better as a pure TD game. Hybrids like Dungeon Defenders aren't really the same thing.
I didn't find it repetitive, but they do a good job of mixing up the levels and enemies so there's always something new to deal with. Some of the levels in Defense Grid are crazy huge, with twists and branches and overlapping paths and height differences. It gets a lot crazier then cheaper games, which are just a flat path. In the end though it's really about building towers to kill giant hordes of bad guys, so if that's the part you find repetitive then you'll always get bored eventually.
Kingdom Rush on the iPad is probably my other TD favorite.