My current manuscript and I have been in three online writers workshops together. All have had good and bad points. First was the Online Writing Workshop for Science Fiction, Fantasy, and Horror. The name may have been a little different back then, but the website looks much the same. The experience is worth sharing with other writers - I'll do the same for the other workshops I've tried soon.
The most important this is being able to find active members, and this is easy at OWW. Start by looking at the most recently published stories - though just reviewing these isn't your best bet if you want helpful critiques. Look for stories with at least a couple of critiques. One might be an accident, but if a story has several, probably the author critiques other people in turn. You could critique this story - but better yet, read the critiques already there, and when you find one you like, see what that person has written.
Unfortunately, OWW isn't ideal for a novel. To keep things fresh, you can only have three stories or chapters up at a time. Given the erratic schedules of those of us who love to write but have to work at something else to make a living, its hard enough critiquing partners on the same schedule to make this work. I did benefit from this workshop though - I received a lot of help with my first few chapters. If chapter one is confusing, you can't blame the fact that somebody missed previous chapters.
Also, you have to pay - $6 a month, unless you go for a longer plan. We all know how that is - you think you're going to stop for a few weeks, then it turns into months, automatically deducted from your credit card while you don't have time to log on. Its a good workshop, and they have to pay the bills, but I always find that racket annoying. I like workshops with a free option, even if they have to charge for premium. You can find a greater variety of critique partners - although I never had a problem with this in practice.
I'll post a few more details if they come to me, but its been quite awhile since me and OWW were together.