Scribophile is kind of a gem as websites go. I caution serious writers to watch what you post. Don’t post complete works that you hope to publish one day. However, as a place for learning the craft, this website rocks.
It offers robust membership numbers, and, unlike some sites, this one boasts of lots of serious authors who know their stuff or are hopeful to sell their stuff, usually both.
The critique interface is smooth and easy to use. The premium package seems like a luxury to me, but it also has some pretty sweet features. By critiquing other works and commenting, the user earns Karma points in order to post their own.
Most importantly, the feedback is really quality so far. I’m coming down on the side of for it. Stop by and give it a look.