It's somewhat common knowledge that you don't ever want to loser power or shut off your console while it's receiving a firmware update as it can corrupt the unit, rendering it inoperable. That said it's not much of a shock to see a lot of people having their brand new Wii U
consoles turned into expensive doorstops when they lost power downloading
the Day 1 firmware update for the Wii U. It's unfortunate to see people having this experience on the first day of a console launch given the hype and anticipation around the launch of the device.
Then again Nintendo had to expect something like this to happen as the majority of the online functionality wasn't available until late
Saturday afternoon. The best advice I have for ensuring that this doesn't happen to you is to hook up your Wii U to a decent Uninterruptible Power Supply
(UPS) while you are doing your updates (which you should probably be doing already). It might also help to have one for your internet modem as well as some people have reported that losing their internet connection has bricked their console as well (which seems odd to me personally).