This would be an amazing game if it was called 24.
1st of all lets talk about the main new feature in Conviction, Mark and Execute. Simply put can mark 1-4 targets depending on how far you are in the game, and in order to unlock the option you must perform a melee takedown 1st, and this allows you to "execute" The execute results in Sam quickly disposing of his foes very quickly, with no chance of them to respond. Its very fun, but can make the game very easy too. I really recommend playing the game on Realistic difficulty because otherwise you are unstoppable. Although even at realistic difficulty the game isnt too challenging.
Now Splinter Cell Conviction is only a Splinter Cell in name only, which is not to say its a bad game, but if you are expecting alot of patience, and rarely using a gun, well you will be dissappointed. This game is Splinter Cell on crack, and Sam Fisher is a action hero who shoots 1st and asks questions later. The story is pretty mediocre, and not very interesting. Thats about the par for most Clancy games though so I dont think anyone here plans to buy the game for the story.
What you do get is some very fast paced gameplay, with alot of cool and flashy moves. You break necks, kill alot of soldiers and evil Russians or evil Americans. 1st all let me mention the gameplay is rock solid and very fun, which is good news. I completed it on the most realistic difficulity in one sitting in about 6-7 hours. Its short, but sweet (could have been much better though) The levels are mostly varied, and allow you to tackle your enemies in a variety of ways. Take the pipe up to the second floor and come from behind, attack right at the front door and make as much noise as possible. As mentioned before stealth has taken a backseat from previous Splinter Cell games. Thats not to say there is no more stealth in Conviction, but its much easier to just go in guns blazing because Sam Fisher is Jack Bauer's brother. he is a fast and very deadly killing machine, with a bad attitude because his daughter was murdered. Dont worry not a spoiler you find out 10 seconds after the game starts.
Now the multiplayer
Its good, but after you beat it there is no replayability in terms of the coop. The coop itself is very well done, and playing with a competent friend makes it very fun to play, and planning accordingly is very rewarding in some very satisfying kills. It was about 3-4 hours long, and provides a prologue to the main singleplayer, which is a good way to provide some backstory to the game. I recommend beating the Coop 1st and then hitting up the Singleplayer. There are other modes too, but I didnt find them very appealing, feel free to read Gamespots review because they cover it very well.
The main problem I have with the MP is that there is no real VS mode. Spy Vs Merc is sorely missed. Obviously the new gameplay would have made Spy vs Merc impossible, but its sad that Ubisoft didnt revamp it and make it work. the only true 1v1 is Spy Vs Spy with enemy AI in the game too. its a sort of who ever gets the most kills in the time period wins, but you can kill eachother for points too. Its interesting at 1st, but loses its appeal very quickly.