I've tried using my cell phone and my car GPS on my handlebars, but the trouble is that neither of their screens are really visible in full sunlight. They are designed to be in shade which is ok if you're inside a car or can hold your phone in a shaded position but not on a bike handlebar. Also, keeping the device fully charged while riding is a bit tricky. I have mounted a small solar panel with its own internal battery onto my handlebar with a rain-resistant cell phone mount:

Then I run a USB cable from the solar charger over to a handlebar bag, where I keep the phone. If your phone is visible in full sunlight, you could mount it in a second waterproof mounting like this one and run the power cable to it. You can find these mounts on eBay or Amazon. They mount with a ball-joint, so while riding you can change their orientation to the sun.

I also have a small bell with a compass built on top of it, from Nashbar, for a general idea of which way I'm headed.

I'm interested in a new GPS device that was a Kickstarter project, scheduled to be on sale soon, called "earl". They call it a backcountry survival tablet. The great thing about it is that the screen is a different kind, one which is visible in full sun. Its features make it the Swiss army knife of hand-held GPS devices. Read about it here at Earl - Backcountry Survival Tablet.