We've had 2 sudden power cuts here this afternoon.
I was in the middle of lining up an edit when everything shut down.
After power was restored I booted up FCPX and got a warning screen telling me that Final Cut Pro X cannot open - To restore the corrupted Project from the backup, click restore. You can also manually restore Projects in the finder'.
It showed the project I had been working on was corrupted and also when it had last been backed up.
So I simply clicked Restore and was back working.
I probably lost about 10 minutes work.
What impressed me was how seamlessly the backup restore worked.