One advantage of using a DSLR over a webcam is that you'll have higher resolution.
Anyway, Mitch actually has an on the spot comparison in the video (3:29) above where he switches from DSLR to his Logitech webcam and there's quite a difference in that the webcam portion is very washed out. Not that it couldn't be fixed by covering his window where a lot of light is coming in, but it's an interesting comparison. Whenever I use my webcam I have to fiddle around with the lighting, and a little bit of light can make a big difference.
There's some great tips in the comments/conversation after this article and video on Mitch's blog as well.
Have you tried using your DSLR as a webcam? We'd love to hear your experience, tips or warnings. Comment below and tell us about it.