Yesterday I had an unrecoverable file system error on my MacBook Air running 10.8 Preview 2. Disk Utility instructed me to back up what files I could and then reformat. Since I had an up-to-date Time Machine backup, I wasn’t worried. I reformatted, and reinstalled the OS. However, I discovered that the OS installation had left me with a copy of 10.7. Would I be able to successfully restore all of my 10.8 files onto 10.7? Probably not. (Many of the settings have changed a lot.) So I decided to complete the installation first, upgrade to 10.8, and then recover my files.
Here’s the tip – something I forgot to do which caused me to waste time. While I was reinstalling 10.7, I created my normal “Geoff Arnold” user account. That was silly, because eventually I wanted to restore that account (apps, settings, files) from Time Machine. I should have set up a disposable account called something like “Super User”, performed the upgrade to 10.8 as this user, and then restored “Geoff Arnold” from Time Machine. As it was, I had to juggle accounts before running Migration Assistant: create “Super User”, log out, log in as “Super User”, delete the “Geoff Arnold” account, etc.
I actually ran into one more problem: trying to restore across the LAN didn’t work, because Migration Assistant hung while looking for computers. So I copied the backup sparsebundle to a USB HD, and restored from that. From checking the Apple Support discussions, it appears that using Migration Assistant with Time Machine is (still) mostly broken.