The benefits of this WinUSB backend is that it will work for 64bit Windows like Vista and Windows 7. The WinUSB backend will not support isochronous transfer. So it is not a complement replacement of libusb-win32 yet.
libusb-winusb will be integrated to the main libusb-1.0 tree (using git). Once it is mature, I think it will be very good replacement for libusb-win32. More backend will probably be added, like the HID backend (HID support) and the libusb-win32 device driver backend (isochronous support, Windows 2k support). When that happens, it can replace libusb-win32 (0.1 stable branch and the 1.0 development branch).
More information about libusb 1.0 Windows backend: