Did a bit of research and found out rebooting in recovery (you have to root your phone and install ClockworkModRecovery, see here for a foolproof howto:
http://galaxys2root.com/t-mobile-gal...y-s2-sgh-t989/ ) and fixing the permissions fixed pretty much all my issues, namely battery life and apps freezing. The process can take a while so don't panic if the progress bar moves slowly. After that, make sure to go over 2 or 3 full discharge-recharge cycles.
Don't use any of those so-called battery savers (JuiceDefender, etc.), they don't play well with ICS and actually drain the battery (at least that was my case). The stock ICS power saving is doing a good job and the auto-brightness usually works well for me.
Right now, my phone has been on for over 12 hours and still has 52% battery. That includes all day (8A-8P) on 4G with regular browsing, social media, email, sms, some music and streaming and a few calls, including a couple of hours linked to my car's bluetooth handsfree thingie. Not bad, I'll be drained before the battery

Some apps can kill your battery life though, such as the live navigation.
The only three bugs of importance are:
- the media player randomly loses focus and pauses the music, same thing with streaming (via TuneIn). Very irritating.
- the inability to place a call from my BT handsfree when the phone is locked, meaning I must do that manually, which isn't very safe (picking up incoming calls is OK though)
- the reverse compass polarity (but OK in Google Maps, go figure).
I'm going to flash another ROM soon and see if it resolves them.
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