There was a SPICE for X11 release a while back that supposedly frees SPICE from the KVM chains... even though SPICE was not designed to be a general purpose remote display protocol. I don't know what state it is in. I don't think any distros have picked it up yet and I don't know anyone using it. I would like to hear a report.
So far as SPICE with KVM, I've used it quite a bit over a LAN and it works fairly well. It is the best free remote display protocol I've found so far and audio works, A/V sync works (although I don't try to do anything other than short Youtube videos), and microphone input works. I believe they have added USB to it but I haven't looked into that yet.
An alternative is xrdp which uses a VNC-to-RDP setup. That gives you reasonable speed over slower links but no usable sound / video.
On the non-free side, the upcoming NX 4 from NoMachine seems amazing. I've tried a few of their previews. It is still a bit buggy but sound and a/v sync seems to work fairly well with less bandwidth and more latency than SPICE seems to tolerate. NX 4 will supposedly have a Windows Server side and a Mac Server side but so far they have not released any previews for those so one has to wonder if they have even started work on them yet and if so, how far they have gotten. I saw "coming soon" type articles about a year ago but I think NX 4 is still a long, long ways away.
I was kind of hoping that Red Hat would buy NX and open source their NX4 protocol... but maybe that is hoping for too much. :)
BTW, SPICE (and the free-cost edition of NoMachine's NX) are both used in a commercial desktop virtualization product named VERDE from Virtual Bridges... and Red Hat is also using SPICE in their Red Hat Enterprise Virtualization for Desktops product so I would imagine there are some actual paid deployments. I'm sure tons of roll-your-own-for-free users are using SPICE in the wild.
I primarily use SPICE with Fedora for a Linux Systems Admin class... giving each student a KVM VM to be root of with reasonable GUI desktop access for general Linux usage. One thing I like about SPICE and KVM is that you can give the VMs private IP addresses with the VM host system having a public IP... and the VMs are accessible via a port on the host system. I'm not quite sold on general purpose VDI / virtual desktops but in this case, it works quite well.