What about remote X?

Spice is pretty 'mature' by Fedora standards in that it's been there for two whole releases now. =) It's the default for newly-created VMs in F16, and it works very nicely for 2D graphics and sound. 3D passthrough isn't done yet. Hans de Goede has a very shiny set of improvements to USB passthrough he's working on which build on SPICE, which have USB 2.0 support (so more devices work and transfer speeds are way better - he has webcams working at full speed inside a VM) and a neat optional feature where if you plug a USB device in while the VM window is active the device goes straight to the VM, but if you plug it in while any other window is active, the device goes to the host.