For the five or so months that I have had my phone I have not had a problem. But I listened to all the buzz about ICS and had to have it. The upgrade went fine, but since then the phone crashes periodically, or just locks up. I am NOT a heavy user with a lot of downloaded apps and widgets. I listen to music and use the phone, and occasionally browse the web.
If anyone knows how I can go back please let me know. Unless there is something you really need in the latest release I wouldn't do it.
Same boat here upgraded to ICS a month back the system wide font used on the S2 seemed blurred I could never adjust to it. The battery gets too hot! (yes, you can burn your hand). Also as widely noted the battery life is horriable. Long story short after eye straining I went back to Gingerbread, and all my issues were sloved.
Stay aways ICS if you own a S2 untill Samsung does some major patching like a proper font, and the battery issue! (there are probably other things as well).
Just to toss out the famous reply by some who may reply with well I did a reset of the phone after upgrading to ICS and everything works.. (that doesn't work)!
Had the problem with Sleep of Death and rebooting after installing ICS. Removed Easy Battery Saver and Easy Task Killer and used the OS custome power saving options. Problem solved and battery life is good.