I had a solid 5 bars when I tried the update. It seemed to download the update OK then said it would be 2 minutes or so to complete the upgrade but the MiFi rebooted in only about 5 seconds so I figured it didn't get the upgrade. When I checked, sure enough, the upgrade didn't take. I tried several times with no success.
I haven't yet tried pulling the battery and SIM as suggested by Jim_in_VA
but, based on others who tried doing this, I am not holding out much hope for success.