User Agent testing sucks, but sometimes you need it for subtle things. In my case I was using it to adjust what I was showing for keyboard shortcut keys (Command or Control). Nothing super major.

if (navigator.userAgent.indexOf('Mac OS X') != -1) {
  $("body").addClass("mac");
} else {
  $("body").addClass("pc");
}

The statements in there use jQuery to add a body class, but that’s not required, you could do whatever.