If you think tower defense games force you to be too passiv but at the same time you like the concept, then this is the perfect game for you. A TD and FPS hybrid...sort of a FPTDS.
The balance is great (and can get quite hard in single player), the towers make sense, the weapons make sense, the creeps make sense, everything in this game makes so much sense. There's nothing in here that is useless or not done right.
It also features a co-op mod up to 4 players. But I prefer the singleplayer:
It's just a great feeling seeing a group of enemies having breach your defenses, slowly walking towards your core and the only thing that could stop them is you and your weapon. You're desperatly trying to shot them, but they just keep on walking.
And only seconds before they reach the core, you manage to kill the last of them.

I started to play this mod when it was in Beta. And heck, I still enjoy it. This mod does everything right. I wouldn't have payed for it, but when I see the price they want for such a relatively bad mod such as "Dino D-Day" then heck, I would rather pay for this.
About the game itself; Well, non-class based but rather equipment based manshooter that needs quite the teamwork and skill if you want to do it right,

One of the best Sandbox'ers out there. But there's one big flaws that get's in the way of fun: The Game.
It tries to much to be a game with balancing and restrictions and whatnot.
But that can easly be removed by mods and trainers. And if that's done, then the exeperimentation can begin adn the fun can not be stopped.