Whilst I havent had one fail yet once running I have found that sometimes getting it to start in the first place can be sometimes a challenge.
1) I upgraded one 7.5 SP1 system, rebooted as requested (presumably .NET 4.5 forced this as I didnt observe during the beta) and went straight into CC to do the MDM/Upgrade. Said the connections string wasn't correct even though having checked it, it was!. I did an IISRESET and tried again and this time it was OK and MDM'd OK.
2) Another attempt. Got a strange "String not set to an instance of a string" when pressing teh Start upgrade button; must be ONSTAIOAO's cousin! Again IISRESET and it then worked.
3) Also noticed that the connection string field on the Upgrade page really serves no useful purpose as you CANT edit the connection string directly there. Should either be read-only or not there at all. Instead it seems you have to edit the connection string in the propoerties of the framework itself before kicking off the upgrade.