I've rarely used Xen
...Just the odd VM here and there. I did run a Citrix XenServer some time ago. Turns out that taking snapshots for backups was a bad idea. ISTR it was all LVM snapshots, but XenServer never actually removed them - just took them out of the admin console. The whole thing got slower and slower as the backup chains got longer, and it took up more and more SAN space. Eventually (I suspect after 256 snapshots), it just refused to snapshot any more.
That was the point where I finally got funds to put in VMware ESXi 4.1, so that shows you how long ago it was. I sincerely hope it's improved since then.
The Xen Hypervisor itself was pretty solid though!