I'm using Serendipity. It's the ROM that lets my phone last the longest between charges. Unnamed is stable, and I might switch back to it for NFC (Serendipity's author traded away his galaxy s2 so I assume he'll stop developing for our phones). Cognition is pretty good, but lacked a couple of things I liked, like having a flashlight toggle in the notification screen.
I want to try CM7, but they don't really have a stable version. I think their nightlies are more stable than the posted stable version. Also, I heard there is an issue with the camera.
miui is getting to be popular, but when I first started, I saw a couple of stern warnings that it is only for experts since that ROM could possibly brick the phone.