I've just upgraded to the AOSP 4.2 Demo.
I upgraded from the 11.09 Defy (+) Quarx 2ndboot build
I'm using a Motorola Defy - MB525
I installed with a 'dirty' install (no wipe of data or cache)
The battery was at a good 85% when I went into the bootmenu.
Here's some things that you may want to know, if you're gonna do this as well:
FIRST BOOT: :fingers-crossed:

The first boot stopped halfway through the animation then went back to the Motorola logo.
This second boot worked, then ran the 'Android is upgrading...' dialog.
While in this dialog, there was a red LED the whole time.
The optimizations of the apps went by quite fast (faster than usual).
GENERAL:
Very, very smooth.
Very fast. Really fast. Like, amazingly fast. WAY faster than ANYTHING I've used (including the new Quarx builds)
SD card is not available, so I can't take screenshots, and the gallery autocloses because of this.
There's no option to connect to a Google Account, so Gmail, Google sync, Play Store all don't work.
Haptic feedback (vibration) works.
No sign of Messaging app whatsoever. But it would be useless anyway because the mobile network doesn't work (at least for me).
There's a red LED no matter what, even after reboots. Battery percentage doesn't affect this.
Battery drain is completely normal, maybe a little better because of no mobile network connectivity.
LOCKSCREEN:
New slider, can unlock in any direction but no shortcuts to Google Now or the camera.
The lockscreen at first is quite laggy.
Widgets work well, but can be a little laggy
Only the digital clock widget works.
Camera will activate but will then close because of error.
CAMERA:
Does not work: 'Camera error - Can't connect to the camera.' The camera then closes, so I can't even test out the UI.
NOTIFICATION CENTER:
Works.
The toggles work, as well as the two finger swipe.
WIFI
WiFi works, however the icon won't change from the 'poor' connectivity.
DAYDREAM:
Daydream works, but because of the gallery not working because of no SD, I can't test out the picture ones.
MOBILE:
I can't connect to my mobile provider, but it could be because I live in New Zealand. So I can't test out 3G.
BROWSER:
The stock Android 4.0 browser works fine. Really fast.
Chrome is surprisingly really fast, but because of no Google sync, Tab Sync won't work.
MULTITASKING:
Works really well, like REALLY well, also looks really fluid because of the new animations.
GOOGLE NOW:
Really fast!
Google Now doesn't work because it doesn't show up. When clicking on the icon in the App Drawer, it brings up the search screen instead.
Voice Search doesn't work
Phone Search works fine
Typing search works fine.
GESTURE KEYBOARD
The keyboard works fine, along with the new emoji keyboard. HOWEVER the gesture typing does not work at all, even when enabled.
CLOCK:
Widgets work fine, even on the Lockscreen
Timer works
Stopwatch works
SOUND:
I don't think sound works.
Don't have access to music, so I can't test that.
I'll update as I find out more.
Hope this helped!