Actually with the barrage of revised and really sharp zoom offerings by various manufacturers, the old debate of better IQ of primes vs zooms have really narrowed to a close. I for one cannot tell the difference if one is taken by a zoom or prime. For me the idea of choosing one over the other doesn reside much on IQ arguements ( but thats for me ).
DonnyDan has cited the classic example of Nikon 14-24mm zoom that puts the 14mm prime lenses to shame.
My main consideration is actually prime lenses often offer ya a larger aperture compared with zoom lenses.
Not so obvious differences: You'll notice that prime lenses often have a much bigger aperture like f/1.8, f/1.4 not found on zooms. This is very useful for subject isolation (bokeh / blurring of background) which is often what many portrait photographers desire.