I have a Dell Inspiron laptop which came with a 1TB drive with windows 8 installed. I cloned this onto an SSD drive (drive C) but left the OS on the SATA drive (now drive F) as a failsafe should the SSD drive fail.
I've just tried to do the 8.1 upgrade. The first attempt failed and it reverted to 8.0. The second time, I initially thought that it had upgraded the inactive OS on drive F because it had changed the F drive to C, and the SSD drive showed up as E. Unable to find any way to revert the drive letters to the way they were, I removed the SATA drive. It then took over half an hour fiddling with the bios before it would even boot (kept telling me there was either no hard drive or no OS). When I finally got it to boot it seemed to be upgrading, but it once again failed and reverted to 8.0.
All in all 4 hours wasted.
Some people may be upgrading without problems but too many people are having problems. Better to wait until Microsoft get themselves sorted or Bill Gates gets a brain transplant.