I don't write Horror (unless Dark Fantasy counts), however I joined Critters several years ago. I wasn't impressed. I liked the way it was set up, the fact that they did a good job of spelling out how things worked, etc, right up front. That was very positive.
I didn't mind having to earn the right to get something critted by giving crits, but when I finally got comments on my work not a one of the 22 crits I got even came close to hitting what an editor rejected it for.
The reason I quit it is because I went through almost three months, reading through the number of things they sent me to crit as well as the ones I was sent and, frankly, there was so much shit that I couldn't find anything to comment on. Well, not without getting into a litany of basic information (this is how a sentence/paragraph is formed--I'm talking basics) or having to give a crit longer than the story itself. It was highly frustrated and I finally couldn't take it anymore.
I know that some people really like it and found a place there. It just wasn't for me.