FWIW here is my experience with GB to ICS upgrade on my stock GB note, (never rooted).
Updated my unrooted SGN via Kies update here in UK. I didn't factory reset before or after upgrade.
Post upgrade, battery life appeared to be a little worse (previously battery life was great under GB bearing in mind that I keep bluetooth off, only switch GPS on when required plus keep WiFi off during the day when I am not connected to my home WiFi router).
I found that Swype was broken
and had to revert to Samsung keyboard.
So I contacted Samsung support and was advised to factory reset my note now running ICS.. Before I get flamed, yes I know about the bricking issues. BUT I have kept the Samsung support email advising the factory reset which apparently fixes random Swype upgrade issues when moving from GB to ICS. I will keep this email just incase I experience any bricking issues in the near future.
So I crossed my fingers
and factory reset my baby (via settings> back up and reset> factory reset), which passed off ok. Then uncrossed fingers and proceeded to re-install apps, set up my email accounts etc etc
.. what a task!
Following that I left the phone on charge overnight and woke up to find a very hot phone, and also found that the charger just about managed to keep up with the overnight battery drain. So I was a very unhappy bunny contemplating reverting back to GB and leaving what was an excellent phone well alone. By the way CPU Spy (brilliant App in my opinion) showed that the phone had spent a considerable part of the night running at 1400MHz rather than being in Deep Sleep mode. No wonder the battery had suffered .
So decided to switch the phone off remove battery and switch on again expecting to see continued excessive battery drain plus lots of 1400MHz action. But no, everything now appears to be normal
With my hindsight goggles on I am trying to remember if I switched the phone off after I re-installed my favourite Apps following the factory reset. I don't think I did, therefore the phone had never been restarted following the reboot that takes place during the factory reset.
The bottom line is that my phone is now running ICS. Swype is fine. Battery drain is the same as or maybe slightly better than before the ICS upgrade given that I am running the same Apps after upgrade as before. So I am currently a happy bunny again
My only other observation is that my SGN would appear to use a lot more battery with WiFi enabled than my old HTC Desire. Therefore I recommend the use of the Llama App to enable automatic switching on WiFi while at home but switchin it off during the night.
I hope my experience may be of some use to someone. I must say that my GB to ICS experience has been one of frustration and disappointment.. But now that my Swype and battery drain issues appear to have gone away, I am glad that I made the move to ICS and am once again very happy with my micro PC