Having been a huge fan of the series (and therefore pretending double agent never existed) I was very sceptical about this one as it seemed to branch away from the formally stealth driven gameplay. However all that stealth etc still exists you just arn't 'forced' into it.
Run and gun does work and can be a bit of fun but you will undoubtedly get more enjoyment out of the stealthy approach. Some of the stealth hand to hand takedowns are pretty cool, for example: one of them has you shoot them in the back of the knee, as they drop, another round in the head... satisfying ;).
Mark and execute is an excellent addition to the game and when you first read about it may appear a bit too 'easy-mode' however in order to unlock an execute you must complete a hand to hand combat move. Depending on the weapon you can mark and execute up to 4 enemies at once.
Last known position feature is another great addition to the game. When you are spotted by an enemy a silhoutte of your last known position appears and this is where enemies will charge, flank and shoot at, knowing this position allows you to out flank enemies and set up ambushes with remote mines etc should you have them.
The single player campaign is short but very good and without a doubt has plenty of replayability. The multiplayer campaign (Prologue) is also very good and about the same length as the single player campaign and some of the sentances that come out of the two characters (Archer: Third Echelon(who sam used to work for), Kestrel: Voron (International version of Third E)) are nothing short of comical.
Other modes, known as deniable ops, include Hunter which is where you play through the coop maps but rather than being objective based you have to take out all hostiles (like terrorist hunt in rainbow six vegas etc) the twist being that if you are spotted reinforcements arrive (from all directions) making it harder. Another game mode is a survival based game whereby an EMP is in an area from one of the maps and you need to protect it from waves of enemies which gradually get better equipped and harder to kill up to 20 waves. During the game you cannot restock ammo or gadgets so have to quickly scavange between waves. Having played a fair amount of these game modes with a friend of mine they are very good and again have a good amount of replayability. With a variation of weapon and gadget choices you can mix up your play quite a bit.
I have not experienced any of the issues that JimSniper has neither has my friend and my machine is half of what his is and I play it on full settings so its not a global incompatability issue with the game. In regards to the DRM it can be a pain if you dont have a stable connection but its the price you pay in the games company's attempt to fight piracy its better than a 3 install cap etc that EA used to spit out on their games.
I can safely say after years of disappointment that Ubisoft has managed to release a cracking game that I will be playing for ages yet without being bored(especially if they keep throwing out free new weapons ;D).