Glasses are right behind hair as an accessory. And they do transform the look of a person. As always, the dark shades are the hippest thing going.
The biggest problem with glasses is that they block our view of a person's eyes, particularly the small fashionable ones. This is hard to solve unless the person just takes them off.
Next, the glasses create reflection. Play with the angles so the glasses don't reflect light into your lens. If you are shooting with on-camera direct flash, this means you have to have the person face slightly sideways.
Bottom Line: What kind of glasses matters, as does no glasses.
105mm, f13, large studio softbox to the right (which you can see in the mirror glasses)