As I've previously written about here, I'm using the Citrix XenServer. I've been using the free version in a production environment for a few years now. Sure, there were minor issues or hiccups, but no deal breakers.
I'm still running XenServer 5.6, although XenServer 6 and even 6.1 were already released.
Why didn't I upgrade yet?
Well, for one there is the mantra of "Never change a running system." Other than that, what's to be gained from an upgrade? For one the list of features in the free edition of XenServer has remained more or less constant.
On the other hand due to changes in the XenTools in 6.1 a lot of problems were introduced for Windows VMs. There are a lot of problem reports on the citrix forums, so I decided to wait until those were addressed.