I have been using Garmin GPS for navigation, both car and bike for many years. I prefer to not look at it much and rely mostly on the audio prompts. After I get the audio prompt I confirm it with a glance at the screen. My current GPS is a Quest and I'll be upgrading to a Zumo this spring. Best screen viewing is when sun is behind me, worst is when I heading into the sun.

 

Garmin 760. No real issues with seeing it.
Ratkiller

 

I have the Nuvi 550, and have no problem reading the screen in bright daylight. It is also waterproof like the Zumo, but lacks a lot of the bells and whistles that the Zumo does. It lacks the hefty pricetag too.