A brand new beta Ice Cream Sandwich ROM has leaked for the more adventurous types, and is probably already making the rounds at XDA-Devs today.
The update is dated January 29, so it's fairly fresh from the oven, and the best improvement over the previous ICS leaks for the Galaxy S II is the battery life, which has apparently increased significantly.
Performance is otherwise said to be on par with the official ICS build, marked as I9100XXLP6, while this one is marked I9100XXLPB. Modders say it is stable enough for daily use, so you can try at your own risk in the source link. Watch a short video demonstrating its behavior below, although it is nor exactly prepared in the best of lighting conditions for its purpose.
source: SamMobile via SammyHub