I'm on my fourth ROM now. I started with Perception (nice), then Cognition (okay), then Serendipity 5.12 (great), and yesterday I flashed Di11i's 7.1. I think Serendipity looks the most formal and was an overall great ROM. I stayed with it for about a month and decided I just wanted to see what else was out there. As I said, I'm just starting with Di11i's, but so far it looks amazing. It has a lot of the same fixes as Serendipity (i.e. BLN, Voodoo sound, RAM increasing, etc.) but I think it looks a lot more futuristic, if you know what I mean. One difference in these two is that if you are to flash Di11i, it gives you the ROM, his custom kernel, and his modem as well. I don't know how I feel about that. I like the ROM, but the kernel and lagfix didn't seem very snappy. I went back and reapplied my new Onix 2.0.3 kernel with SGS kernel flasher (amazing app). I also am not very happy with the Di11i modem. It seems to be lacking quite a bit more than my old one. I'm going to give Firefly and Phoenix a try, but may very well end up going back to Serendipity.
As for kernels, I've run the stock, SpeedMod k13c, Paragon6 (a version of SpeedModk13a), Di11i's, and now Onix 2.0.3 (a version of SpeedMod k13d). I was very happy with the SpeedMod k13c. It was plenty snappy and made my battery go about 18 hours on about 40%. I'm now on Onix which in my opinion, is a little bit faster, but drains battery quite a bit quicker as well. Although I am on the 1000 Hz option, rather than the battery saving 100 Hz. I would suggest one of these two as a good base kernel though.