This really depends on the premise.
No I would not leave a site over a single bad review. If they reviewed my favorite game unfairly I would be a little disappointed but reviews are the personal opinions of the reviewers and can differ depending on the tastes of that reviewer. Of course reviewers should try to keep their personal bias to a minimum when reviewing however their tastes are not something they can cover up. Maybe the reviewer who wrote the Zelda review and MW3 review for example simply prefers FPS games. They can try to give Zelda a fair unbiased review but in the end they didn't enjoy the product as much, so their review isn't flawed.
However if that review goes over board. Which 1Up is notorious for doing, where the reviewer cusses and insults the product and its developer. Then that is grounds to stop reading their reviews if it continues. I'm not talking one bad review where a writer freaks out and goes off the handle. One review is understandable and hopefully the website reprimands the reviewer for unprofessional conduct. But if the problem continues which it has on 1Up as long as I can remember from them officially calling the GameCube a little kids piece of shit or talking about how crappy Miyamoto is. To the more recent personal attack on Gearbox over Duke. If the website doesn't correct itself and act in a professional and acceptable manner then people should not continue reading.
Lastly bias, if a site becomes known for always trashing a companies product. Say a site is anti-Microsoft and no matter how great the game the website gives them a low score. Say Halo 4 comes out and everyone gives it a 10/10 but their site gives it a 7/10. The site does this on a regular basis against Microsoft then maybe gamers should leave the site, honestly who wants to go to a game site where the staff rail on one of the big three or any publisher for that matter. Its okay to have bias but you need to act professional and to always give a company a bad score regardless of quality is bull shit.
I personally left 1UP because of their poor journalism. They went over board constantly they acted highly unprofessional and at least at the time I left they were heavily anti-Nintendo. Other sites like GameTrailers.com I left because even though they are slightly better they are paid off, they give better reviews to companies that sponsor them and while they don't give terribly bad reviews to companies that don't it is very evident that they do not treat them fairly. Example the VGA's have you seen a company win an award at the VGA's who wasn't affiliated with the VGA's? Recently I watched the preview for the VGA's on TV the show had Pachter on with Geoff and they talked about the five nominees, at the beginning they mention all five games by name including Zelda:SS but then Geoff asks Pachter and himself to go over each of the five games. They go through four of the five games and refuse to mention Zelda at one point Pachter even states that four games are amazing even though five were nominated. A major reason for this is the fact that Nintendo doesn't write a cheque to GameTrailers and Nintendo doesn't reveal exclusive content during the VGA's like Sony, MS, EA ....etc....etc... so Nintendo gets thrown under a bus. GameTrailers doesn't want a Nintendo title to win because Nintendo isn't paying them.
One last factor would have to be the websites community (Forum) honestly if VGChartz began writing biased reviews on a regular basis and even begun acting unprofessional I would have a harder time leaving because the website has such a great community. Fact is I am friends with several members and a writer or two. So despite my grievances with a couple articles or the actions of one or two staff members I would not leave the site as long as the community remained!