I've looked at
www.1x.com and they have very tight controls. You have to upload an image (only 1 per week on the free account) and this has to be approved before it goes live on the site. You can upload the image, but you can't ask for it to be reviewed until you've left critique for at least 3 other images. As Abers mentions, this leads to the temptation to enter some dross just to get your own feedback which kind of defeats the object (and I would always wonder if the comments were sincere of compliance dross). So far, I have 1 image on their and it's not been looked at yet as I find their system even more time consuming than PR's.
I think there may be a veiled compliment for this site in there somewhere
And Cathus, totally agree - while it's nice to get 'nice shot' comments, I also like to get the 'what if you'd tried this' or 'I don't like XYZ and think you could have ABC'd to get a better...' kind of comments.