My first review of a game so bare with me.
Gears of War 2 is better than the first. I believe the first Gears of War was more of a "test" so to speak, like an introduction. To be honest, I believe the creators of the game probably didn't realize how huge this game would become. So enter Gears 2 with bigger levels, better graphics, more enemies and more action. The campaign is very compelling, although it's the same video game cliche of 'One mission just leads to another' - it's still a very interesting storyline - especially the emotional plot of finding Dom's wife. The introduction to new characters is fresh and the duo between Marcus & Dom is very believable. On the easy and normal difficulties, you should breeze through the game with no problems. When you get to hardcore and Insane difficulties, some might have trouble.
As for the co-op, the added feature of Horde is a nice fun way to sit back and kill baddies with some friends (or strangers if you prefer). The combo of fun, intensity and insanity rolled into one makes a great co-op experience. Taking photos now is fun also, gives you something to do when you're dead awaitng the next round (which I do constantly). As for multiplayer, there are some complaints with glitches, bugs or whatever - I'm sure these will be fixed in due time.
I remember a certain website that recently changed it's layout that had similar issues that were quickly fixed, remember? So be patient...