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 seamless 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.
e) The biggest benefit with VMPlayer is seamless file operations between host and guest. You can copy, move, delete files without any problem. VirtualBox still don’t have this feature (till Version 4+, May 2012).
f) Clipboard on VirtualBox is buggy and most of the times non-functional. VMPlayer has a reliable clipboard and you can freely move data between host and guest.