depends on three things:
1) facial features of the guy (shape of face, etc)
2) what kind of facial hair he can grow (full beard, only goatee, etc)
3) personal preference of the woman
depending on the shape of your face, certain styles of facial hair will ENHANCE your natural attributes and make you more attractive. other styles will bring focus to negative traits and make you look less than great. for example, although i look good clean shaven, i look great with the "2 day unshaven" setup. i also look great with a goatee, minus the mustache. but at the same time if i just grew a mustache id look ridiculous. theres websites on the net that give advice for each head shape... google it.
as far as what girls like... i read somewhere that when girls are ovulating and at their peak state for becoming pregnant, they are even more attracted to the "rugged" or "masculine" features than usual. for instance, everything else being equal theyd be more sexually attracted to the guy with the five oclock shadow than the clean cut preppy looking guy whos clean shaved. for what its worth, ive always had better results with girls when i have some degree of facial hair.
-SLB