In my experience, some webfonts look okay at smaller sizes but most of them don't.

Typekit shows browser samples that you can break down by OS as well. ie. http://typekit.com/fonts/adelle-web click on the 'Browser Samples' tab

Look what happens when you change the h1 on custora.com from 50px just up to 51px on Chrome [Win 7]: http://dl.dropbox.com/u/4129016/50-51.png (it looks a lot better)