Maybe not.  But more reviews does equate to a larger sample size,  a broader based look at even more people's opinions and can provide you a much clearer picture of the general reception and opinions of a game.   It certainly isn't the end all, be all.  But it usually provides a solid metric or form of measurement for reviewing games.

The only major issues I have with these sites is they incorporate the fluff departments (IE: Xbox Magazine/Playstation Magazine).  These sources are on the said consoles payroll and therefore hardly ever give completely accurate, unbiased reviews,  and some of these only review games for certain systems (360 only or PS3 only,etc) which is a one-sided way of looking at things.

It's still the best metric available for gathering widespread opinions about a certain game.  It doesn't guarantee that you will like a game,  but I would say almost any 90+ rated meta critic game you can certainly see WHY it achieved what it did (Even if you didn't enjoy it).   (For example,  me and GTA IV)

Metacritic gets a lot more reviews that most appear on Gamerankings a few days later. Gamerankings and Metacritic are own by the same company. Gamerankings for some unknown reason does not include all the reviews that are on Metacritic.

I take the Metacritic ratings quite seriously in making my purchases.  Like most adults I have a job and kids, and therefore a limited time to enjoy my hobby (or my wife would say addiction) of videogaming each night after the kids have gone to bed and before I need to.  I also hate wasting money on a game that I buy and then have to drop playing because of a lack of continued motivation to play on.  So I prefer to weed out the undesirable games before I decide go out spending my money.  Metacritic is the most valuable tool for me in that pursuit, but I use it in combination with my genre preferences.

Let's say I am about to buy a game and I know I like JRPGs, so something like Blue Dragon (Metacritic Score of 79) is a good buy, but I don't like Survival Horror quite as much, so Left 4 Dead (Metacritic Score of 89) is not for me.  If Blue Dragon was at 74, and Left 4 Dead at 94, I'd probably have gone for the latter instead.  Without Metacritic such a decision is not possible.

Still doesn't matter.  Larger sample groups do not determine which game I would like and for anyone else for that matter.  You did say "opinion" which is a contradiction to a "solid metric or form of measurement for reviewing games."  And if I understand your later point correctly, I will agree that gamers will acknowledge a quality game regardless of their personal preferences.  But one game with more reviews from one group than another still doesn't not equate to being more accurate.  Just the one with a more popular opinion.

I stoped caring about metacritic when Gears2 got 93 it deservers much less.

Most of these reviews are from reviews made months before the game is even launched eg. Halo 3, GTA IV, Kill Zone 2 are perfect examples of this. How can reviewers possibly give the games fair and accurate reviews if the online component in complete games has not even been tested on a wide audience? Betas/demos are not the full game and reviews based on game demos is a joke.