I probably base 80% of my purchases on previews rather than reviews. It usually works out reasonably well, with a few duds here and there. However, if I do actually read a review for a console game, it's usually either IGN, Gamespot, or Joystiq. Portable is basically 100% TouchArcade. I should probably branch out a bit on that one. And I should mention PVCF as well...FarCry 3 is an example of a game that I hadn't paid any attention to and purchased almost entirely due to people's comments.
Mostly Yahtzee from Zero Punctuation. He cuts out all of the crap and gives you an honest review. If something is garbage and not worth your money, he'll just come out and say it. He doesn't sugar coat anything. He's not influenced by advertisers. I've had good luck with the games he recommends. His reviews are also usually pretty funny. I also go by what other PVC'ers are playing and recommending. I've gotten shafted by Gamespot quite a few times. I'm pretty sure their reviews are bought and paid for by advertisers. Anything with a lot of hype always get an 8.5+ score. I also can't say that I've had much luck with what users at Gamespot recommend. The PC gaming crowd over there all thought STALKER was the greatest game ever, when in reality it's a piece of shit that's not even worth the hard drive space or time it takes to install.
IGN user comments, 1up and this and another forum I go on. 1up are top notch and unbiased like IGN though.
Recently, I've relied solely on chi's recommendations for my 360 purchases. Thus, I haven't bought any 360 games for quite awhile.
I don't have the time to take that chance. I only want to play the best so I'll buy games rated above 90/100. Sometimes I'll dip into the 80s.
I usually just check out the metacritic and gameranking scores. If I have the time to read a review, I usually go IGN.