For the business person, the answer is that Xen is a safe, stable, well-tested choice for virtualization which is used by industry giants (Amazon, Rackspace, Verizon, etc.). It has a robust consortium of companies behind its development and it has the price, performance, and security to go toe-to-toe with the best offerings in the industry. Plus, it has a proven 10-year track record which includes powering some of the largest clouds in the world.
For tech-savvy users of F/OSS, however, there are additional considerations.