I've had ICS for a few days. I did the update manually, by using a pre-rooted FF18 install from XDA. I did the version that resets everything, so "do a factory reset" doesn't apply to me. (I DID, however, restore the user data for two apps: ConnectBot and Google Authenticator. All other apps were restored without their data.)
My battery life is, to be frank, terrible. I drop 1% every 3 minutes when the screen is on. (It is set to the lowest brightness.) It drains much less when the screen is off, but a phone without a screen is kind of hard to use. If I don't get a fix for this pretty soon, I'll have to re-ODIN back to Gingerbread and restore my old nandroid to get Blazer 4.1 back.