As far as I know, there are currently two JB ROMs floating around on Android Central, XDA and other sites. They're a bit crude at this time, with camera not working and other features like MMS needing third party apps or other workarounds. Give it time.
I've seen a few ICS ROMs and even tried a couple. Some features like camcorder require workarounds, but all in all, they work well. I switched back to a GB ROM I like (Gingersnap) when I was having severe overclocking issues (I later learned to be careful with that). Between ICS and GB, there are things I prefer about both. ICS is quick and smooth, and has a slick interface. The dialer and contacts are cheap and ugly, however, compared to my current GB ROM. And the blue text is a bit hard to read. My current GB has some of the stuff I do like about ICS, so it's the best of both worlds right now.