XenServer is my favorite virtualization product. After previewing Vmware, Xen, and Hyper-V, I found this to be the easiest. You don't have to know linux to use. I like that XenServer can import virtual machines from any of the other hypervisors. I use scheduled snapshots and archiving, to where I don't have to worry if my machines are backed up. XenMotion is a breeze and makes it extremely easy to upgrade or change out servers. I also like that you can have heterogeneous pools with XenServer Advanced or above.
Room for Improvement:
You have to set limits on scheduled snapshots, otherwise it won't let you take more. I have a limit of 7 days, with daily snapshots. The free version is somewhat limited in features. It is best for testing or small organizations. Something that might be a con is that when all of your servers are virtualized, you may not need some of your IT staff.
I use this to host all of my servers and my virtual desktops. This has saved my organization a ton of money. Although there is a high initial cost, it has paid for itself in the long run. If you are planning on doing a VDI solution, I would recommend using the same company for virtualization. For XenDesktop, use XenServer; for Vmware VDI, use ESXI. To test the full features of XenServer, try out the open source equivalent- Xen Cloud Platform.