Through Homebrew Channel.
Loading ROMs from USB doesn't seem to work at all. So I copied a few to the SD card, and tried them out.
Sailor Moon (arcade) - works
Street Fighter 3: Third Strike - exits back to Retroarch menu
Super Street Fighter 2 Turbo - works
The Last Blade 2 - crashes, with randomly colored pixels on the screen
I tried using the sideways Wii remote as a controller for Sailor Moon, and it didn't really work. The D-pad worked for navigating the Retroarch menu but not in the game itself. Also, the + and - buttons (mapped to start and insert coin) did not work in the game. I was able to map the A and B buttons to start and insert coin, but it was still pretty useless without being able to use the D-pad. So I plugged in a Classic Controller Pro, and that seemed to work perfectly. By default, only the D-pad on the Classic Controller worked for movement, but I was also able to map the left stick to movement. Given my experience with Sailor Moon, I didn't bother trying to use the plain remote for the other games.
So... it's a little rough around the edges, but it basically works.