I'm wanting to do it on a heavily customised site, a lot and I mean alot of changes to the TPL's including the smarty logic on them, it's got changes on the the order screens, the config database, func.php has been changed, config.php changed.
I think it's going to be a case of 10 gallons of coffee and a day to do it
Think I'll start all my new sites on the 3.5.X branch even if it is unstable.