iStache has the most mustache friendly features of any Mac software to date. Take that, iWork and iDVD! There are twelve varieties of mustaches available for use and each of them have a handy name (many denoting the person who made that particular mustache famous, or infamous as the case may be). The names of the mustaches are: Bushy, Squirrly, The 6:30, Thin Curls, Thick Curls, Ben Affleck, Straight Gayness, The Hitler, English Gentlemen, Fuzz, Poe Poe and Got Milk?
In addition the the large variety of mustaches, you can also change the size of the mustache to better fit the face you are giving it to. This is done by a very easy to use sliding scale set between 1% and 100%.
Placing a mustache is easy too. All you have to do is select the mustache you want from the pull down menu and click on the spot you want to place it. However, this part can be a little imprecise. You can't move a mustache once you've placed it (you can undo it and start over though) so it may take several times to get the placement just right.