In Praise of Scribophile

I recently discovered Scribophile and cannot recommend it highly enough.
For those of you who are unaware of Scribophile you can link to it here.
So what is Scribophile? It is a website for writers. The way it works is that when you first join you can connect with people and critique their work. You can select stories to read that are from many different genres, short stories or chapters of novels. Every time you critique someone else's work you receive Karma points. You then build up Karma until you have enough to post one of your own works for other people to critique. Karma points awarded increase the longer the critique that you give but on average I get about 1.5 Karma points per critique. You need 5 Karma points to post one of your own pieces so for every three to four pieces you critique you can then have one of your own pieces critiqued.
Why is this better than other writing groups? I think it is better because everyone has to give before they can receive. I am always surprised by the other online groups that I am a member of, where people post stories and thank you for your critique but then claim they are too busy to critique your work. What makes them think I have nothing better to do that to encourage them in their writing? In order to improve we all need to get feedback.

Once you join Scribophile you can have quite a lot of interactivity for free. If you want to have more than two pieces live on the site then you need join. This is relatively cheap and with the exchange rate at the time it cost me £45 for a year's membership.
I am currently editing last year's Nano novel and the feedback I have had from the chapters I have posted so far has really helped me to develop the story and give me some confidence that people are going to want to read it.
Also I have started critiquing a story and it means I am getting to read a really good novel before anyone else.

Have you used Scribophile?

What other sites/groups do you use and are they as useful?