Mark Harmstone has been working on a Btrfs file-system kernel driver for Windows. I wrote about it back in February with it being a kernel mode driver for Windows that offers Btrfs read and write capabilities. Since then Harmstone has continued working on it and he's now released a new version of the driver.
In an email to Phoronix he explained this new version includes major speed and stability improvements, support for partition-less volumes, and RAID support. The speed improvements are referred to as going from "sluggish" to "blistering".
There is also asynchronous reading and writing, Windows sparse file support, security improvements, and various bug fixes. The RAID levels currently supported are RAID0, RAID1, and RAID10.
More details on this Btrfs v0.5 Windows kernel driver can be found via Mark's GitHub.