I regularly use all four of the O/S's and was a beta tested for Longhorn/Vista
Two years ago I was converted to OS X and still consider this to be the best of the bunch. It's limitations are that it can not run many of the programs I must use. Consequently I have Parallels installed and use XP Pro as well.
Linux is good but just not in the same league as OS X or XP.
Poor old Vista! Destined to be thought of as another ME?
In too many ways simply not better than the O/S it superseded.
I hope that MS get back on track with Windows 7 but I will not hold my breath