Normally I hate it when somebody posts a response here which says "Why don't you Google it?". However, I feel compelled to offer this advice because searching for 'Corsica GR20' brings up plenty of personal experiences.
I've found that this site gives me a good impression of the walk. (I've got a Mountain Leader award but I reckon that I'd be 'out of my depth' for much of the GR20). Click on the 'day by day' links, at the bottom of the page, for comments on the conditions. Click on the thumbnails for larger pictures:
http://homepage.ntlworld.com/phil.newby/corsic a04/index.htm
Chris
PS: If you click the 'Home Page' link and look at the bottom left of the page, you can find an e-mail address for the couple who own that website. It looks like they're keen hill-walkers, so they'd probably be happy to share their experiences with a fellow enthusiast.