I have never used any of the 2.1 versions so I can't say much about it. I have used 2.2 beta1, beta3, beta5, beta 5.5, and beta7.
Beta7 seems fairly good to me and I have not experienced any major bugs. It is based on the leaked Froyo ROM so naturally that ROM is not 100%. That being said, the only bugs that I have noticed are; No haptic feedback on the android keyboard, the arrows in the 3g indicator dont light up when sending or receiving data over 3g.
LauncherPro is the only launcher on it (TW has been disabled) so for some reason you cannot install updates of laucherPro.
Over all I have been very happy with using Cog. Up until I flashed it, I was very frustrated with my Captivate. I was ready to get a G2, unlock it, and sell the Capt on ebay. Then this rom came along and I tried it. Its a whole new phone.
If you are going to get into rom swapping and havent done it before. I recommend reading the Cog. FAQ and the threads on it so you have a good understanding of what you are doing before your do it. You could always try the 2.2B7 and if it is too bugg for you, flash down. Or you can flash 2.1 and if you like it and are feeling adventurous, give 2.2 a try. Always make good back ups and have Odin One click and CWM on hand in case something goes awry.