I was, but only because I was clagging together a build cluster from old machines, and the company wasn't prepared to spring for VMware. And having got that build cluster working, Xen decided (in their less-than-infinite wisdom) to remove high-availability failover support from the free version, which was really the main reason for choosing XenServer in the first place.
If I was still maintaining this, I'd probably be changing to Oracle's VM Server, which still uses Xen under the surface but doesn't feature-cripple it. I don't understand why the XenProject guys would want to crap on their users like that. I don't mind having a slightly less spiffy UI and less of the "pro" features, but surely high-availability is the single most important reason for running a cluster. Lose that and you lose the whole point of it.
But since my company got bought out, everything's moving to a nice new server running VMware, so I don't need to worry about it any more.