Snow Leopard to Mountain Lion upgrade via USB drive: Early 2009 Mac mini
The Mountain Lion download from the MAS to a cloned external FireWire drive took about an hour, creating and copying to a 16GB USB stick took
maybe 15 minutes (
thanks Macworld!), reboot, upgrade from Snow Leopard using the new USB stick installer to the external FireWire drive took
maybe a half hour (wasn't paying attention) and probably another good hour and a half until Spotlight finished on the upgraded FireWire drive. It's now running as fast as Snow Leopard ever did, and all of my critical applications are working normally (Lightroom, Logic, my FireWire and USB audio interfaces, etc.).
I'm one happy camper.
Now that I'm convinced it's a smooth process I'll squirrel away my additional bootable Snow Leopard drive, and then either clone the external FireWire drive with Mountain Lion back to the internal drive, or (more likely) run the upgrade again against the internal Snow Leopard HDD tonight.
Good times!