It's...Left 4 Dead, with more weapons and unique zombies.  I picked it up for $33 bucks off Steam, so given that first impression, I can't complain too much.  I mean, that's an expansion pack level price.  My friend Joe and I played through the first two campaigns, we haven't touched any other multi-player modes yet.  The level design captures the feel of the area it's based on, so it doesn't feel the same as the original, locationally speaking.  Characters chatter between each other to help move the narrative along, which seems a bit stronger.  What else...oh, samurai swords.  The new melee weapons are effective in close quarters and makes deciding between secondary weapons a tough choice, especially with no familiarity with what's ahead.  I hope the "new" content becomes more apparent as we continue through the game, as of right now I wouldn't call this a sequel.