As someone using all 3 OS's day to day (Ubuntu is my primary development OS, I have an iMac next to me, and I switch my laptop to Windows 7 for video games in the evenings), Windows 7 is easily my least favorite OS, it's performance is generally crummy compared to OSX and Ubuntu, and it's UI often aggravating.
See I find the opposite is true for me. I don't have much experience with OS X but between ubuntu and Windows 7 I _much_ prefer Win7. I just can't stand how much screen space is eaten up by useless padding, menu bars, sidebars and panels, not to mention the overly large fonts. I realize that everything is customizable, but I don't have the patience. Windows 7 also seems faster and more responsive to me but responsiveness is largely subjective and situational.
