My S2 is not even a month old....the ICS OTA came after about 10 days. Battery life was noticeably less. Well, I did a reset..moving apps to SD...yet no measurable difference. .Android OS still ridiculously high. I searched and found two helpful posts for a noob such as myself. Ignoring all the rooted phone solutions, I was left with a factory rest at DN 2's direction and also draining and charging the battery so that ICS could "learn" the battery. I can't recall the post in order to give due credit, yet it was real simple: 1) discharge the battery to where the device turns off; wait a while...then turn it back on and let it discharge again...do this a couple of times until you cannot turn the phone back on. The battery is now completely drained. 2) Take out the battery...wait 10 seconds or so, and replace. 3) Plug in the charger and allow the "off" device to completely charge. Unplug the charger...wait a few seconds, then plug it in again and allow it to completely charge again..still in the off position. 4) unplug the device, turn it on and enjoy.
Well, the above battery adventure did appear to work, yet Android OS was still a major drainer. So, I was left with doing DN2's factory reset. I followed this to the "T", yet did not save any apps to my SD card....wiped SD and phone completely...phone was new, so nothing to lose.
I then set up my emails and went into "Play" and put back on only the apps I totally trusted and actually intend to use. Finally I went int to the settings as DN2 suggested...made changes and disabled apps I knew I would not use.
Like I said it has only been a day, yet my, my, my what a difference so far....Also, lagginess is gone! It is as fast now as it was with GB.
Dark Night 2's advice seems, at least for me, to be spot on....there are other solutions (xda developers) for those of you that are rooted and know all about kernels and flashing the rom flavor of the day.
If you are having issues....take off your photos, sync your contacts and then do a factory reset and reformat the card. It worked for me! The true test will be how the battery is working in 4-5 days.
Next I want to do this on my wife's phone...she and I got one at the same time. ( I went from a BB and my wife was on a Samsung Seek) My daughter was up for renewal too, yet she went to the dark side...peer pressure I guess.