Pheace: I usually give it a month before giving a game a serious look review wise(I'll take forum experiences over reviews any day). Before that people are just too happy that they even get to play the game to realize any possible flaws.
Given the media barrage Witcher has had I'd certainly give it at least a week.
Right, I predict the majority of "reviews" shortly after release will be bullshit, regardless of the source. People will either completely overhype it, or bash it for objectively irrelevant details.
It will probably take a while until the hype subsides and some useful reviews start coming in. And I'd always recommend not to rely on a single source of information.