I run in what I'd say is a small shop, we have 21 sites the smallest of which have two hosts 13 'guests' with 12TB of shared storage. Our average site has four hosts, 22 'guests' and 48TB of storage (almost ready to double those as they are nearly full). The two data centers each have 11 hosts, over 100 hosts (virtual sprawl I'll admit I'm losing count and that's a bad thing) and 120TB of storage each.
For VMWare with that many nodes we were looking at hundreds of thousands of dollars that no one wanted to spend.
We switched to XEN roughly two years ago, in that time I've had one issue that I was literally able to 'google' the answer to before our Linux vendor responded to the support request. Yes you can get support for XEN from any of the big guys, and guess what, at least for the distro we use support for XEN is included. For us VMWare couldn't touch that with a ten meter cattle prod.
Oh I manage all of that with one 'console', and no not one per site, one console to rule them all (ha had to sneak that in there).
Was it free as in beer, sure, but that caused us to learn what we run rather than just poke buttons and call for support the instant something (anything) goes even slightly awry as I've seen others running competing solutions do.