Upgraded to ICS (O2-UK) the day it came out and overall it seems good. Chrome Beta is great, with lots of useful goodies if you use chrome on your pc, but has frozen a couple of times. Other than that it seems to work great, face unlock is a bit of a gimmick (if you're trying to show it off to people, try not to do it with a big grin on your face, or you'll look stupid when it doesn't work). Performance seems good, browser feels faster, but that's just a general feeling and the built in data monitor saves downloading 3rd party apps for this (yet to see how accurate it is, but it seems roughly in line with the My O2 app, allowing for a few days difference in start date). Quadrant also shows a 20-25% increase over 2.3.5 (I can upload screenshots if I can work out how).
I know the update is non wipe, but just to be safe I did a full system wipe before the update (possibly unnecessary, but its a good excuse to get rid of unused apps and should prevent most of the issues some people have reported with ICS, having to resort to a factory reset to improve reliability and performance.
Hope someone finds this useful