I've been running the Windows 8 Consumer Preview for a few months and although I'm okay with Metro replacing the Start Menu, I hate seeing the new interface by default every time I reboot. When Windows 7 starts, you hit a login screen (assuming it's enabled) and then you're brought straight to the desktop.
When Windows 8 starts, it displays a lock screen that you have to move out of the way before entering your credentials, and then you have to dismiss the Metro interface before accessing the desktop. Like I said, I'm cool with Metro, but I have no desire to see a full-screen Start Menu when I log into my PC.
Hopefully the retail version of Windows 8 will make it a little easier to force Metro into the background, but it's entirely possible to do this on the prerelease builds if you have five minutes to spare. Likewise, it's easy to disable the lock screen and have Windows automatically log you in for hassle-free reboots.