If you like the tower defense, if you like classic stick shooters, if you like trial and error defense tactics...
Buy this game.
pro tip: For playing with your steam friends, someone has to start a game and set it to allow friends. Also you may have to allow it to accept "anyone" if you have issues with the friends option.
This game is absolutely worth 2.99 if you have at least 1 friend to play it with.
My opinion is that in it's current rate of content and enjoyable challenge I'd say its a fair game that if you like the genre is worth 9.99.
Game sounds are a bit generic, but they aren't expected to be stellar yet and for the price you get enough.
The animations, while repetive are very smooth and mesmeric when you enter the Flow state of mind, and become completely mentally engrossed in trying to keep yourself along with your team and your defenses off the ground.
I recommend it for harcore zombie killers who want to vegitate, and for the casual pick up and drop occasional scream fest for anyone.