I just signed up with PICtage a few months ago. I originally heard about them a couple years ago from my step sister who's a wedding photographer in Virginia. She was not pleased with the print quality back then and went back to her local lab. Gary Fong and Denis Reggie both highly taut PICtage (of course they are part owners) and claim to use them for their weddings. It was worth the $250 sign up fee (credited back) to see if they'll work for me as I have spent way too much time editing and fulfilling my own events. I have a couple events that I've since posted, one bride said she liked the site and found it easy to use. I have yet to get any prints from them yet so not sure how that will turn out...
Everything else (besides print quality which I'm waiting to find out on) has been a pretty positive experience, customer service has been great and they've helped me through a few issues that I've had.
I suppose only time will tell, but this is indeed a great idea.
No one has mentioned them yet, but another online proofing site is
http://www.collages.net/ I have not tried them yet as the verdict is still out on PICtage, but thought I'd mention them anyway...