I used the Windows "XP Mode" VM on one laptop we provided to a customer, who promptly changed and forgot the wrong password and can't seem to figure out how to launch their applications and also get those applications to talk to the connected network.
I realize this could simply be one bad experience coloring my opinion, but it was a strongly negative experience.
Conversely, I purchased VMWare Workstation with my own money and have used it extensively for years. I can send a removable HDD to a customer, have them run VMWare Player, and be reasonably confident they will be able to use the software successfully. I recently did this with a non-English-speaking customer and they had no trouble following my screenshot instructions.
In my opinion the tight integration between host and guest OS in the Windows built-in VM is a drawback because it's hard to figure out the line between them. VMWare is a more clear "here's the VM, running in its window" distinction.
Your mileage may vary, of course. Lots of people prefer VirtualBox as well. But I'm a VMWare adherent and will continue to be.
