My first post was very biased, but this one will be a bit more serious. Infinity Ward takes about three months to fix one glitch, and those updates create more problems in the game. Also, Activision is just going to have another Call of Duty game released ths year (They have released one every year since CoD1), so a lot of players will likely migrate over to CoD7.
The matchmaking is also quite laggy, because Infinity Ward uses its own servers. After you prestige once in online, it gets very boring. It's the same thing every time you play. However, after prestiging once, I started to use different weapons which made it a little different. It still got boring to me very quickly. This game also has the most horrific spawn system I have ever seen in an FPS. Another thing, the game is always free-for-all. It doesn't matter if you're playing Team Deathmatch, it's still Free-For-All, don't let the titles fool you. All people care about is kills, so in objective games, don't rely on your teammates to take a position.
Basically, if you enjoy glitchy run and gun games, buy Modern Warfare 2. If you enjoy well designed FPS games that require team work and have amazing vehicular gameplay, buy Battlefield: Bad Company 2. (You get ALL of the DLC for BC2 for FREE. MW2 has five new maps coming soon for 1200 ms points....)
I know it's between CoD5 and CoD6, but in my honest opinoin, they are both terrible games.
lol Don't take my advice, I absolutely hate this game, and have nothing positive to say about it. Watch reviews and gameplay videos on Youtube, and you can probably get a better idea on which one you want.