Where to start, I guess the obvious thing to say is that its awesome and that you should buy it ;) Also, for those wondering about performance, I could run the game on my gtx460 at the "extreme" setting and had no problems.
The single player is great fun, there are a few key differences from Crysis however, the most plainly obvious for me was that there isn't so much exploration any more. In Crysis you could explore the map in any direction but only certain areas had any actual gameplay in them. In Crysis 2 you are directed through the maps in a linear fashion but each area provides many different routes and ways to get through it.
In each area the game just lets you tackle it however you wish, and gives you a bunch of toys to play with. Cars can be kicked at things and off ledges, explosive barrels can be hurled, and just about any solid object can be moved around as you see fit. It's a playground of carnage and its fantastic :)
Guns! They all feel great to use, really meaty and the customization from the first game is still there to modify your weapons on the fly. A new feature is the ability to customize your actual nanosuit with upgrades also, such as bullet deflection, threat tracers, proximity alarms, reduced energy usage for modes and an awesome "air stomp" that will take out multiple enemies if you aim it right ;)
Story is serviceable for the most part, there are a few WTF moments but its mostly enjoyable, and its presented very very well, always from first person. There is some slight retconning from the first game but nothing major. A few characters from the first game seem to have gone missing without any explanation however :P
The one thing that is kind of a letdown however is the enemy AI, far too many times they will remain stationary and not attack, or get stuck in some sort of animation loop bugginess. It also has the ability to amaze however, some of the alien enemies will jump from floor to wall to ledge to another wall then slam you in the face and its very very impressive indeed.
I've been playing the MP also, not really enough to offer a decent opinion but I'm liking it so far, its very similar to call of duty but with added nansuit powers to mix up the gameplay a bit.
One final thing I'll add, I really love the sense of movement the new cryengine manages to convey with its motion blur. 99% of games I've seen motion blur in seem to apply it to the entire screen no matter what is moving and what isn't. In Crysis 2 motion blur is applied to objects relative to their movement speed as viewed by your player camera. The effect is fantastic and really sells the visuals.
Screenshots (Could be spoilery! You have been warned!)
http://steamcommunity.com/id/darrenweekes/screenshots?tab=public&showdate=1&filter=app_99830