I noticed immediately there is a lot more battery drain than before. The culprit is Exchange Services, which keeps on waking the phone very often. I have set my Exchange account syncing option to sync every four hours, but it seems to work in real time push mode instead. In battery stats I can see that the percentage of battery used and wake up time for Exchange Services increases steadily.
I did a quick google search and found out this problem is major PITA with Nexus S and ICS. Can't say how disappointed I am. Only workaround I've found is to manually disable Exchange Services in Settings-Apps.
Seems that this upgrade is not for features or usability. It is solely meant to circumvent Apple patent lawsuits and is absolutely rubbish inside. I don't want to go into deeper discussion of bugs and worsened features I've discovered. This email battery drain thing is definitely biggest showstopper in my case. I really need my corporate email from Exchange server and disabling ES is not a feasible workaround.
Question: Is there an easy way to downgrade an unrooted SGSII to pre-ICS firmware?