Web users judge sites in the blink of an eye: Don’t think design matters? This study shows that users take a grand total of 50 milliseconds to come to a judgement about your site.
Lindgaard and her team presented volunteers with the briefest glimpses of web pages previously rated as being either easy on the eye or particularly jarring, and asked them to rate the websites on a sliding scale of visual appeal. Even though the images flashed up for just 50 milliseconds, roughly the duration of a single frame of standard television footage, their verdicts tallied well with judgements made after a longer period of scrutiny.
Via Slashdot
This is easily believable, although I would say that visual appeal isn't all that makes a good website. I rank highly a few other things: