You know the old saying: something that sounds too good to be true...is.
Webcam eye tracking has been touted as a sort of redeemer: you don't have to pay those ugly meanies at Tobii, SMI, ASL, etc. those exorbitant prices to track the eye with expensive cameras, extra infrared lighting, fancy monitors, and elaborate software. Easy as you please, you can just use the camera embedded in your computer and voila! -- eye tracking results at Wal Mart prices.
Well, Aga Bojko has done us all a great service by conducting practical tests with webcam eye tracking and exposing these fallacies. Her results are here.
It's not so much a scathing expose as a simple reminder that you get what you pay for. As Aga points out, webcam eye tracking can indeed detect the gross regions where the respondent is focused, but not much else. If that's what you need, go for it.
Most of us need more, and more precisely. She rightly finishes by saying "it's a nice quick-and-dirty option to have handy" (or words to that effect), but note that the list of limitations is quite long! For example, low frame rate, poor accuracy, head movements harm results, static pages only, short sessions required, and so on.
More time in market will help. For the moment, webcam eye tracking is a work in progress.