Macs are actually a interesting concept, since they all run in a single hardware(which spares a lot of developer headaches) and they're indeed visually appealing. Never tried them.
Windows, like Pilchard says, is Windows. People use it, because it has the software they were taught to use..I have XP in dual boot, mainly due to some games and/or photoshop.
Linux, depends on the distribution. Most common ones, stand tall near both Mac and Windows. Also, if you really need it WINE, is becoming quite strong, actually and if it doesn't work my XP VBox, is normally up to the job. Either way, after a few months on linux, you start to dislike windows, and a couple more later, you start to understand why command line based software should be the only kind of software(with it's due exceptions i.e. browsers/graphic manipulation/some games).