I have been running iOS 5 jailbroken without issue. I just attempted to install iOS 6 GM for the first time via Opt + Upgrade. (I have not installed any betas). The upgrade failed with error 6 and this wiped the phone. The phone would now only boot up into DMU mode and needed a full restore. So - if you are running iOS 5 jailbroken, it might be sensible to first Opt + Upgrade the phone back to a non-jailbroken iOS before you attempt iOS 6, and in any case *make sure you have a full backup of settings*.