My impressions...
A few buttons/toggles look different... No benefit nor degradation
Google Now... I find it hard to believe that this took an OS upgrade... Frankly, a good combo calendar/search app would be just as useful to me. I guess it is a minor benefit. I don't know if it was in this forum or elsewhere that I mentioned it, but I see little of interest in the current version of Google Now that isn't possible with the previous location aware Google search capability. We'll see if they expand the capability in the near future (hopefully not requiring another OS upgrade).
I have read reports of other improvements that aren't really important to me like improved voice recognition... Who knows, I may try this again, but when I tried it before it worked fine, I just haven't had a need to use it.
Battery/temp/performance... Frankly, when I am using the phone, I am usually in an app so quickly that I don't notice any additional smoothness of the launcher, and I never had noticeable battery issues not directly related to playing games for too long... and I never felt the phone getting even slightly warm unless I spent over 30 minutes in a game.
I am happy to have JB, but not thrilled.