TL;DR Metacritic user ratings suck
If you didn't know already, Metacritic's aggregate user rating scores for Modern Warfare 3 and Black Ops 2 are dismally low. This contrasts with the critic aggregate ratings which are generally quite high.
Some people might like to claim that the "players know best" or even go as far as to claim that the critics are bribed. This doesn't mean that Metacritic's widely-renowned user rating is reliable, or even accurate.
There are a few of reasons why the user ratings aren't accurate or reliable and why you shouldn't take them into account. One of them is a slightly psychological view of it and the others are a more down-to-earth explanation.
First off, the psychology behind lots of Metacritic user ratings is to exaggerate it as much as possible. If you look at Black Ops II for PS3's user ratings thus far, you can see that there are a heck of a lot of 0s. Now look at the reasoning. Some of the reasoning doesn't even have anything to do with the game. Now look at the PC version's reviews. Lots of 0s as well, but one of them stands out, with it starting "I'm too lazy to even download the game".
So what you have there is someone (or something, as will be discussed shortly) who doesn't even own the game giving it a review (another factor which will be discussed shortly). This knocks the reliability of these reviews right off the cliff, just from this point it is clear these reviews are poorly thought out.
Another reason the user ratings suck is because of the fact you don't need to own the game to leave a review. That really speaks for itself - it leads to biased and/or plainly uninformed reviews. Springing off of this, old versions of the game may have had bugs which caused people to give such a poor rating. This was the case for Skyrim on PS3 which had the infamous issue with RAM deficiency when a large game save was loaded.
This is a more swanky reason that the Metacritic user ratings are unreliable and inaccurate. It's somewhat clear that the Metacritic user ratings are swamped with bots to upvote bad ratings, downvote good ratings and leave bad ratings. You can see that with the image to the right (click to expand - read both reviews, they're paraphrases of each other), and also on these links - 1 2. These are more than likely bots used to downrate games and not actual humans, or at the very least, one very sad person on multiple accounts.
So in conclusion, it's better to rent a game and see for yourself than base your opinion off of unreliable aggregate review scores. In many cases, the critic reviews are more trustworthy, but you can't ever beat your own opinion on a game to see if you'll enjoy it or not.