I flashed the 4.2.1 beta last night after a full wipe, set up my phone and charged it overnight and have been using it today.
Initial tests seem great, I have managed to get 2 hrs 11 mins screen on time and still have 66% battery left. The jelly bean volume bug seems to have been fixed, it drove me mad that from half way to max the volume didn't change before as I watch videos and listen to music a lot on my phone.
Only issue I have noticed so far is the signal strength, sometimes it says I have full reception then it jumps back down to the normal 1 or 2 bars I usually get at home. Other times when I would get 1/2 bars before it says no signal, I am using the same modem as on Slimbean 3.1 (XXLQ6), is there another modem I should be using with the new version which might be better?
Once the inverted Gapps are back for this version it should be very nice.