Don't like caps but it makes a big difference for me when I'm at WDW. A cap keeps the sun off my head and out of my eyes so I don't have to change from glasses to shades all day long.
I never wear hats except in Orlando during the hot months...It really helps keeping the sun off your head and it is great during rain. I wear a baseball hat and it works great with ponchos and keeps the rain out of your face and eyes....however, I have terrible hat hair so I leave my hat on all day, except on dark rides or rides it will blow off my head...