I was a premium subscriber of gameknot.com for years, and chess.com for approximately the last year. I think gameknot.com has the best correspondence chess setup of any site I’ve seen, though I’ve only looked at a few.
The tournament system and user-submitted puzzles are nice as well, though I think chess.com’s will surpass gameknot’s within the next year. I had recommended many people to GK, some who became paying subscribers.
The best thing GK probably has going for it is it’s playerbase. It seems to be a lot older, friendlier, and more experienced than on other sites which are centered around blitz chess as opposed to correspondence chess.
The same cannot be said about the administration. I was informed that if I upgraded from one paid subscription status to another more expensive one I would be refunded what I had already paid but not used on the first - when I did this, they said there were no refunds, and my first subscription effectively became null with no compensation, and only rude replies when I asked. At least those replies were answered...
Go a year and a half in the future, and I mention in a private message to another player that I also play chess on chess.com. I would never do it publicly for other people to read; I don’t think it’s proper to advertise a competing business on any business’s territory. I tried to log on the next day any my account was suspended, for "violation of the Terms of Use" - which I never violated. They left no indication as to why my account was suspended; I can only assume it’s for the mention of another chess website in one private message. I have sent repeated queries over the last week with no reply.
In summary, I hope chess.com (and other competing chess sites) pay attention to the good ideas GK has had and steals them. GK doesn’t deserve it’s subscribers. I have to give GK one star out of five for Service & Support since that is my only option - if I could put in negative stars I would. Their "service" makes it not worth trying their website at all, regardless of any nice features they have or may add.
My only regret in leaving GK is I have no way of contacting a lot of the friends I’m leaving behind - they completely disabled my account. It’s too bad, really.
Oh, since a lot of people find blitz chess important, I should mention that the blitz chess game on GK is horrible and uncared for. They claim they are trying to improve it, but it hasn’t changed in two years. Forget it.
EDIT: It has been over two weeks, still no reply from GK. Clearly they ignore people like me - they can, they already got my money.
ANOTHER: It’s been months, more messages sent, no replies. They will intentionally ignore you if you have a problem. I’ve fully moved to chess.com... and I’ve discovered [not to promote that site, but to show how much GK simply sucks] that chess.com offers far more features for free than you will pay sixty dollars a year for on GK. I’m pretty sure my number of games are unlimited, and I don’t [yet] pay a dime. There’s reasons that I’m still considering paying for a subscription, which will be cheaper than GK... that’s the way it should be.