Hellgate to Diablo 2 is what Killing Floor is to L4D, but the suckage here is on a whole new level
My impression so far is that it's a good game if you see it as a free third-party mod. Since they're selling it as a full game, I'm going to score it as a full game, and frankly, your $20 could be spent better elsewhere. From what I can tell, single-player is just multi-player with bots. The game opens with no story whatever, not even a cinematic teaser to set the mood. Multiplayer feels so unbelievable for so many reasons. First the maps SUCK. Generic futuristic buildings or open fields. Throughout each round you walk through the boring landscape to the next trader, because apparently killing the monsters makes cash appear in your pockets. At the end of the round the traders is unlocked and then you can spend your money on new guns and ammo, although you get so much money from shooting the monsters (I won't say zombies because there are so many random creatures) that you'll likely only use the trader for guns the first round.
Did I mention this game also has random RPG elements that are completely unnecessary for an online FPS? If you get X number of kills, your gun shoots 5% better, and depending on your class you get a very uncreative boost in something.
This game lacks polish and it's obvious right down to it's horribly bland menu layout. The 5 points it's gotten is solely because I loved Left 4 Dead, and although this game fails at excelling or expanding on this genre, at the end of the day it's still zombie shoot-em-up, which is awesome.
Bottom line: Lack's polish, no backstory or cinematics; plays like a generic third-party mod.