Hrmm. No, I don't think reviewers complete the game always. I think they sometimes do, but I have read quite a few reviews after playing the game where I thought "there is no way this reviewer got to XXXX." I'm not sure how much it matters, but I think reviews have become somewhat of a joke in the current industry anyway.

Depends on the reviews.  I typically find that reviews on gamefaqs.com are pretty good if you take them as a whole rather than individually.

Big gaming site reviews though... nah.  Not too useful.

 Words of wisdome indeed!  I was going to say that, but you beat me to it.

I would say that it depends on the game and it depends on the Genre ...

I would say that reviewers should base their review off of a significant portion of the game, for (semi) linear single player game this means that they should finish the game, and for a conventional multiplayer game they should play every level in every game mode. With games that have 'unlimited play' like sandbox games I think it would be reasonable to say that they should have racked up a pretty decent play time (40+ hours) before they review the game ...

In my opinion, I don't think a single person can review a MMO game as a whole ... I think that you can only review a realm-race-class combo and a decent number of these reviews would count as a review of the game as a whole. Basically, playing a Dwarf/Elf/Human Hunter and playing a Troll/Orc/Tauren hunter in WoW is a similar yet different experience ... In other games race-class combos were completely unplayable and simply by choosing the wrong combo could ruin your game.

Good sites DO complete the game (major objectives anyway) like IGN and Gamespy. They'll usually post a 'played so far' review not to be outdone by other sites, but won't assign a score until they've finished.
You can usually tell which sites do, since they'll bitch about have to pull double all-nighters to finish Zelda or Oblivion in a timely fashion.

In order to review a game the reviewer needs to experience all the aspects of gameplay. In some games, you need to finish the whole game, in others, not so much.

With Bioshock for example, you could get all aspects of the actual gameplay fairly early on and review it based on that fairly well, but you would be missing out on the story which is important in that game. So for a game like Bioshock, you need to finish in order to properly review it.

With a game like Dewey's Adventure you can play it for 2 hours and get all the gameplay. You don't need to play more to review it because there's nothing new introduced and there is no story element that really affects how you play. Even with the different boss and level puzzles, it's still just the same basic gameplay. I'm not saying that's a bad thing, in fact it's a great thing to have simple systemic gameplay that works well throughout the entire game, I'm just saying you can play a few hours of some games and get everything out of it.

Lately I dont care of the reviews . If the game has any interest for me I buy it . And surprise , I only regreted buying in these last few years around 1 or 2 games ...

I have to admit though ( even if Im ashamed ) that once , if GS didnt gave the game a 6 I wouldent have bought it ... that was untill I saw some less than good reviews there , and I decide to not read them anymore ( though I still often read the overall impressions ) .

You can't know wehther the game will lend itself to needing a full playthrough to experience it all without doing a full playthrough.

And just to go with the Oblivion comment above, you could review Oblivion without finishing the game but not Zelda. With Oblivion, the story is not important, that game is all about quantity and nothing about the story. With 10 hours into that game you could easily write a great review and get everything that game has to offer.

With a Zelda game, you could get 10 hours into a game and not be able to review it. You get new weapons, tools, and attacks the entire game that all affect how you play the game so that's one that needs to be finished.

 Then how come I know I've experienced everything in Dewey's Adventure that needs to be experienced gameplay wise?  how come I knew a few hours in Bioshock that I experienced all the gameplay I needed to?  How come in Oblivion I knew I experienced everything I needed to?

 After playing enough games and learning how to look how they built their gameplay it's pretty easy to see these things.  Even with Gears of War, its gameplay is simple enough to know that you've gotten everything out of their mechanics.  I'm not saying that at that point the normal gamer should stop playing, far from it, I'm just saying you're not going to learn anything new and at that point the game can be reviewed.