It's great at what it does. Much more useful than I expected, feels more thought out than some commercial alternatives.

XEN Virtualization with their custom X-passthrough driver is super fast as well. No hardware acceleration of course, but it didn't feel choppy at all. As long as you have enough RAM (I'd say 16GB at least), it works pretty much like bare metal (even though there are several VMs running in background at all times for network, fw, usb, etc), way faster than Spice, VNC, RDP or Virtualbox (talking about UI not number crunching).

That's what I've been wondering too, especially now when more and more applications are starting to use hardware graphics acceleration.

I used it briefly but found interacting with weird USB devices (mainly Arduino and RTL-SDR) slightly too annoying.