I was an avid user of VirtualBox and was pretty happy with it. It gave me freedom to experiment with different OS in a rather safe, transparent, and convenient way. So I used to keep Linux as host and Windows as guest. Performance was good and bugs were non-existent.
Then I found VMPlayer. It surprised me a lot with following reasons:
a) It supported ISO images for Ubuntu, my fav Linux disto.
b) Set up was easier/comparable to VirtualBox.
c) Unity mode: What a feature! A seemless integration of guest OS with host application. You don't have to click the mouse, you never lose window focus. I can now browse through guest OS with Alt+Tab.
d) Very stable, good performance, and I am a happy customer.