I've had a Microsoft Surface Pro 3 now since October 2014. Its pretty great. There's been the occasional problem with the Logitech V400 Bluetooth mouse I use losing connectivity to the Surface, but all in all it is an impressive device.
To freshen things up as I started using the device straight away after purchasing it, I've formatted and re-installed Windows 8.1 Pro so I can get it running 110% . In the process I've found a problem with some of the drivers- Windows does not see the Surface Pro 3 HID Sensor Collection or the Surface Pro 3 Simple Device Orientation Sensor. This is kind of a big problem as with the device in tablet mode. I tend to use it allot in portrait rather than landscape- especially when I'm reading.
What wasn't working (all I tested before finding the solution):
This HID Sensor Collection fault is pretty simple to resolve. What you need is the following Windows update: Media Feature Pack for Windows 8.1 N and Windows 8.1 KN Editions: April 2014 or search via KB article KB2929699. Simply download this patch, install, restart your Surface Pro 3 and voilà!, you're back to normal.