Man you guys must really hate this game. I've already played through it once with my brother and we're playing through it again on contract difficulty now. Fun game if you have a friend to play with. The story is... well, I'm not sure there is a story. But the weapons are cool, there's lots of blood (heads come off if you hit them right), the coop rocks (you actually have to play as a team to survive) and the overall experience was enjoyable (albeit short).
We got it for $30 a piece at tru at launch and I think it was worth it. I wouldn't spend more than $30 for it though since it is so short. It has seven levels in it, and each level takes about 45-60 mins to beat.
By the way, I don't understand the comparisons to Mercenaries 2. That was an open world game which blew big time. This game is very linear and heavily focused on coop.