Only played for 5 hours so far, but wanted to share findings to now.
I read several reviews before buying, and if it hadn't have been reduced I probably would have waited until it was.
That said, it is a good game - I've certainly found it quite addictive so far. You will spend quite a lot of time playing the same levels over again (although there will be slight variations each time in terms of available tower slots, etc), but if you don't mind some more effort than in a lot of TD games, then you should still enjoy the game.
Still plenty that I haven't found, but the game doesn't take long to learn, but getting used to the 'forge' system takes a bit of trial and error to begin with.
Buy if it's on sale, maybe think twice if it's not.