I don't think you have to root your phone to go back to 2.3.4.... You can just flash it back. There's a way how here:
Re: Samsung Galaxy S2 SGH-i777 2.3.6 OTA update no... - Page 7 - AT&T Community Support
I got it from the other thread here about this update. A thread that should be merged with this one maybe... But I am going to think a few days about it, but I might do this flash.
update if anyone cares: Juice Defender is working pretty well. I am getting better battery life now than even with 2.3.4. Not promoting this app, but just saying.