This is good news for anyone who cares about performance of USB connected harddisks / ssds. The old usb mass-storage protocol is well known for its poor performance. UAS however allows NCQ and thus allows effectively using the full USB-3 bandwidth. If you've an UAS capable harddisk enclosure then all you need is a 3.15 kernel build with the UAS driver enabled and you should instantly get better performance. Note that most harddisk enclosures, including USB-3 enclosures do not support UAS, so if you want to use UAS double check before buying a harddisk enclosure.
One use-case of UAS I love is to have a 2.5" ssd with a full Fedora rawhide install with me so that when people ask me about hardware compatilibty issues they are having, I can simply plug in the ssd to there laptop boot rawhide and see if having the latest kernel + xorg fixes things. For a decently priced UAS capable 2.5" hdd enclosure search ebay for: "sedna usb 3.0 2.5 inch hdd enclosure". when shopping for UAS enabled devices, always check that the device description mentions UAS or UASP.
Besides UAS support, the 3.15 kernel also features support for using USB-3 bulk streams from userspace through usbfs. To use this you need the just released libusb-1.0.19 release. One use case of USB-3 bulk streams from userspace is using these for usb-redirection in qemu. With the upcoming 2.1 qemu release this will be supported allowing the use if usb-3 redirection with uas devices from within a qemu vm. This will be supported with both qemu's host usb redirection, as well as spice's network usb redirection. This means that starting with qemu-2.1 qemu's USB redirection features full USB-3.0 compatibility.