It really depends on the viewing angle, the amount of sunlight, and your brightness setting.
I keep my Garmin Nuvi550 set on 50% brightness (mostly to extend battery life) and with the proper viewing angle, I usually have no problem seeing the screen. Of course, if the sun is reflecting right it right into your face, no amount of brightness will help.
Also, some GPS units may vary in their brightness setting. I think most are fine, but my daughter once bought a Magellan for her car and the display was so dim, even at max, that she returned it. The Magellan I have in my car, on the other hand, is plenty bright.
I guess if you can read the display outside while holding it in you hand, you'll be able to read it while riding, too.