Everyone should avoid updating their Samsung Galaxy S2 to the new Ice Cream Sandwich (Android 4.0.X) until you see a steady stream of reports on this community forum that they have fixed the critical battery life issue.
We've (regrettably) updated 2 different Galaxy S2 to this release and the BATTERY LIFE ON BOTH HAS BEEN TERRIBLE since the update.
My wife, who is a modest user, went from getting through the entire day with probably 30-40% battery life left to now getting low battery warnings at around 4 PM. If she uses it much it all, it's sooner. We've adjusted everything to be less active than before ICS (e.g. email being checked less frequently, during more features off, etc.), but it still doesn't get her to 4 PM.
Now, her phone has the new FH13 release that is supposed to improve battery life, be we've seen no improvements. In fact, we're now seeing some weirdness such WiFi getting turned off and on at random intervals. Phone tech support has suggested that we need to go into a store and have their tech guys do a clean reinstall of FH13. What a pain...
If I could go back to Gingerbread, I'd do it in a hearbeat.
(BTW: Android 4.0.X on my daughter's T-Mobile Samsung Nexus S seems to be fine. No battery life issues there...)
After performing a factory reset on my Galaxy S II Epic 4G Touch phone with the S:D710.10 S:FH13 version, most of my problems except one have gone away. My phone's Bluetooth when connected to my vehicles factory Bluetooth keeps connecting and disconnecting the phone. After researching this issue at various forums, tried re-syncing the phone, disabling the WiFi, turning off my wife's phone WiFi when she is in the vehicle, and still the problem persists. The vehicle is a new Honda CR-V and the connection issue was not an issue before Sprint released ICS update. Under Android kernel 2.3.6, it worked fine. Now with Android kernel 3.0.15 (FH13), it does the connect/disconnect every few minutes. My wife's older HTC phone with Bluetooth has a solid Bluetooth connection in the vehicle.
Since we don't have 4G or LTE in my area, cannot truthfully say all my problems have been solved. Since the factory reset, at least the re-installed app's don't go "None Responsive" on me anymore. Battery life, about the same as before.
I like ICS and have it installed on my HP Touchpad. Just hope that when Sprint releases an OTA, they have fully tested it.