Yup, upgraded to stock ICS 4.0.3 last week, horrified at the battery drain in spite of switching a lot of stuff off. Now safely back to GB and yes, nothing in ICS that I miss...

ok...my gf faced a problem on her s2..the phone frequently turn off by itself..its cause by firmware or hardware?

thats weird lor, usually the google contacts will auto sync with google, in settings>account & sync>google there, you try sync your google account and see, maybe the contacts are online and u just need to redownload them/sync them back, if you did not mess with it, all your contacts should be online
turn off by itself or hang/freeze? if turn off by itself, could have a few common problems
1. the power button, long press the power button could make the phone off, if the phone auto off usually when it is in pocket, then this could be the reason. the pocket too tight and pressed the power button accidentally.
2. battery loose, if its loose, put some papers at the end of the battery so it will push against the pins at the top tightly