At the moment I'm using the blue themed Omega 15 rom (Android 4.0.3) along with Siyah kernel on my SGS2. This combination works really great, battery life is much improved compared to the stock kernel and some Samsung bloatware is removed. For me it's one of the best and most stylish custom ICS roms at the moment.
In the past I used Checkrom Revo HD v6 (Android 2.3.6), imho still the best custom rom but Android 2.3. I think I'll switch to Checkrom ICS once it comes out.
So which rom are you using and what's your experience with it?