Worth it for sure!! I love it, and have been playing for ages, multi player is very fun as well, as its up to 4 player co-op. It's always being developed, with nice amounts of dlc, and more being released at a good rate. And i mean, tower defence in FIRST PERSON! That's AWESOME! I think they did it just perfectly too.
At that price I wouldn't pass up the chance. Hope you enjoy it! --- "I'm a warrior! Not a... variety of flower!" - Vegeta