Much cheaper than other major Hypervisors (VMware, Hyper-V).
Easy to install and use at the beginning.
Room for Improvement:
Hard to maintain system because Xen's management tool (XenCenter) isn't very useful. For example XenCenter does not have any orchestration features (as opposed to SCOM, SCCM, etc.. from MS). However Xen provides APIs to develop other tools.
As a result, Xen is cheap but a good value.