I updated last night and have been editing in FCP X all day and haven't encountered any problems. Of course, I backed up my hard drive to an external drive first so if any problems arise I can always boot from the backup and be back in business. I wouldn't recommend upgrading any OS without a full backup but so far, it looks to be working fine. Even DVD Studio Pro 4 is working and that's quite old.
Has anyone updated and had any issues with eSata cards or RAIDs? I upgraded my laptop and all my editing programs seem good but cautious to upgrade the mac Pro until I know the OWC and GRaid eSata Raids won't have issues.