Problems include, Firefox crashing on start-up, popular mobile browser Dolphin not working at all, Chrome is unreliable and unresponsive, Facebook can't post, Skype can't log in, and the System process uses 90% of CPU and crashes. Notifications are now very hard to display, and the system is very laggy and unresponsive.
There doesn't seem to be anyway to undo the upgrade, so at best users will have to do a factory reset, losing all their data.
I for one, haven't worked up the courage to do that, but plan to back up all my data at the weekend, and give it a go. I'm expecting to lose Facebook though, as the latest version requires total access to your google account, including permission to send and receive emails without you knowing, access to your calendar, backed up photos, and any other information Google stores about you.