It was slower that it needed to be, it had annoying long-standing bugs, and after using it for a couple of years I still had troubles finding some or the functionality.
By no means it was bad, for most part v4 was ok and some of it was great. But when people describe it as some kind of perfection totally ruined by the update… I don’t really get what they are talking about.
Unless you speak Korean/Chinese/Japanese, the list is pretty short. DGS for correspondence; KGS and IGS for live. With some effort it’s also possible to play on Tygem and WBaduk.
FICGS kind of does this, but I’m not sure it qualifies as a go server. Even if it does, I wouldn’t recommend it over OGS.