Ok so it looks like there is a slight issue with this on Windows 8.1 Pro (works fine on Mac). I wasn’t able to get the new drivers working with both Serial and USBSerial1 until I compiled this firmware for my Photon:
That said I did get USBSerial1 working, but Serial stopped working (with new drivers).
Here are the new drivers. https://github.com/avtolstoy/particle-usb-testing
These code changes will be released in v0.6.0-rc.2 or if you want to try it now, make sure you follow the section on removing old drivers and installing new ones. Don’t try to do it differently, it won’t work