Pretty poor, right now, unfortunately. I was looking at replacing VMWare Workstation with Hyper-V. However, when I installed Ubuntu Desktop on a brand new Hyper-V VM, I had to do some grub hacking to pass kernel parameters to get the display resolution right. Even after I got the display resolution right, there was a noticeable lag in the GUI refresh. I know that maybe with some more hacking I could have gotten everything to work just as well as it does on VMWare Workstation, but that proves my point. In VMWare, everything Just Works, out of the box. In Hyper-V, you need to tweak things to to work, and it's not clear what all things you need to tweak.

Allegedly that will be improving with Windows Server 2016, which is supposed to have better support for GPU acceleration for VM guests, but I'm waiting to see what actually gets delivered before I get excited.