my phone was JB 3.0.1 under redsn0w. last night i updated to 3.1.2 in itunes without doing a restore. it updated fine with no kinks at all. i jailbreak'd with blackra1n and it literally took no more than a minute.
went into cydia and downloaded all my JB apps, it retain its settings. i didnt have to fiddle with any of the settings. well besides categories, i unloaded everything from the folders i created before i did the update.
only 1 problem i noticed when i was using iphone-explorer, it wont let me go into root ("\"). it says the phone needs to be JB first. that is strange, i know my phone is JB and it worked when i was on 3.0.1. unless iphone-explorer doesnt support 3.1.2 jb yet? i was going to change my dock background this way. i wasnt able to do it so i had to SSH with cyberduck.
from what i can see, battery still seems the same as when i was on 3.0.1. cant tell if its smoother or not.