Mouse and keyboard worked fine for me, although if you go to the settings and set the locale, it seems to trip up on the list of countries because everything goes very slow and jittery. Once you close the list, performance is back to normal again. I wonder where we can provide feedback for this, I miss the smiley face feedback button that you get everywhere in W10.
I wrote an app that listens for messages on a websocket, and when it receives a certain type of message, it plays a random MP3 file from a folder. I had thrown something like this together in WPF a few weeks ago for something and I was wondering if it would work on the Pi. I was super-impressed that I could just run and test it on the desktop, where it functioned exactly like my WPF app, and then deploy it to the Pi without making any changes whatsoever, and it just worked. This is incredible.
One caveat, thought: the Windows IoT preview doesn't seem to contain audio drivers yet, so while you couldn't technically hear the MP3's, I could see that they were being selected and played. Can't wait for the first update. Meanwhile I'm experimenting with an addressable LED strip and the GPIO pins.