ESX, GSX, or Virtual Server?

Hello Everyone,

           I am currently researching Virtual Server products in order to choose which product the company I work for will specialize in selling to clients. Our clients range in size from small local business (10-50 users) to large firms (1,000+ users) and everything in between. There are of course the big three Virtualization platforms (ESX, GSX, and Virtual Server) and I am currently looking through documentation comparing and describling each one. However I also wanted the opinion of experts that have used these products in the real world rather than just the opinion of white papers. Our customers are 99% Microsoft so these Virtual Servers will be running Microsoft Oprerating Systems (2000, 2003 server). Any input on the Pros and Cons of each of any of these platforms is much apreciated! I wll of course split the points with a grade of "A" for anyone that takes the time to provide their input on these platforms.