I too experienced crazy battery drains after upgrading to JellyBean (OTA update, P769 T-Mobile prepaid, pure stock, not rooted). I tried turning off Google Now after reading this thread, but it didn't seem like it helped much.
Last night we decided that my wife is going to use the phone instead, so I factory reset the phone before I gave it to her so she can start over fresh with her google accounts, etc. She left home this morning with the phone fully charged, and when I came home from work in the evening I checked and she had more than 85% left! Granted she didn't use it much throughout the day as she still carries her old phone, but she did use the phone to make some phone calls/chat and take some pictures. Just as a comparison, the last time I had the phone with me before the factory reset, the phone was draining battery maybe about 15%/hour just sitting on my desk.
So, for those of you experiencing battery drains, maybe you can try doing a factory reset if you haven't. I realize this is not something most people want to do unless it's guaranteed it will fix the problem, but if it's as annoying as my experience, maybe it's worth to try.