kept coming up with an error when I tried to update it via a wired network connection, the only way in the end was to do it via usb stick, strangley the manual for my a/v amp suggests updating via usb stick even though it has a network connection at the back.
I did have a Sony bluray player that did update ok over a network connection, took less than 5 mins I think to download, install and restart.
my advice is, if your internet connection isn't that stable then do it via usb.
*music2*
ps. regarding power failures, the chances of it happening during updating are very slim so I wouldn't worry about it.