http://www.penny-arcade.com/comic/2009/6/19/Heh heh.
I think I'm halfway through the game. It's purely awesome and despite your guy being a little psychotic, his intentions are pure...I'm not sure how the story will end, but so far it's trying to stay alive and figuring out more about the virus at the same time. The running up the sides of buildings thing never gets old, and often when you think you have the gameplay figured out it'll throw new twists at you.
There is a large amount of causing chaos, so if you're not the type that enjoys that it's not the game for you...but seriously, why would you be playing open world games if you didn't? :p
I just hijacked my first chopper in the air...that makes getting across town a HELL of a lot quicker

I really doubt I've seen everything the game wants to throw at me, but I am pretty high on the upgraded skillsets...I think. It unlocks more after every mission, so really I have no idea.
IMO if you skip this game, you're seriously missing out. I've heard the Xbox 360 version is the worst one, though, which explains the review scores being slightly lower...glad I got the PC one, but I'm sure they'll update it if that's what you're going with.
On another side note...you can only imagine when they were first thinking about this game, it was a zombie game. Hee.