The original Ao2 was a decent little shooter. I'm not going to try and compare it with the other over the shoulder buddy games - but it's worth comparing with its brethren.
The controls are improved, the aiming is improved, the graphics have been upped and the sound is all tight and impressive. The setting is China, and unfortunately all a bit samey (and doesn't always make a great deal of sense), but they try to mix it up a bit (but no powerboating or parachuting / paragliding this time). The "back to back" sequences of the first game that could be absolute show stoppers (replaying the same 2 minutes 10+ times could sap your will to live) have been revamped and are much improved. In fact there is little that hasn't been taken and polished to some extent.
The weapon system is fantastic. It would be nicer if more "special" items could be found or unlocked with achievements. As it stands most guns can, if all the same parts are installed, pretty much mirror up with a gun from the same bracket. This does somewhat limit the true inventiveness of the process...but with 30+ weapons and hundreds of mods to buy, find and collect you really can have fun working your gun up from its basic form to something you expect to see on "Pimp My Gun".
The flaws in the game are common, but no show stoppers. "Heavies" are occasionally frustratingly located (and ridiculously impervious to damage much as they were in Ao2) whilst you can feel as if you're shooting waves of badguys with no real direction as you're railed from one section to the next. The railing wouldn't be too bad if the plot was strong. Unfortunately it starts poorly and does not improve. The game ends on a confusing note.
Moral quandaries are little other than a quick way to get extra cash, or a new upgrade (making 1 replay of the game a must in order to max out your achievements). They could have been improved, but are really just a very small and unimportant element (if perpetually ambiguous).
There's still much to improve. The game isn't overly hard, or particularly challenging, substituting waves of enemies instead of difficult or intelligent ones. The high number of achievements put into the game do keep it interesting however, and you will find yourself charging enemies down for the sake of getting those 25 close combat kills you need for your 15 pts and a shiny medal.
As a solo player game it's a real waste of time (although the AI has been improved). In the end the game is a very literal action movie for you and one friend to play through, and as such I've enjoyed it greatly. For a buddy action game it's purpose built.