I've been using a Garmin Vista C for a few months now. I don't use it on shorter (20 mile) rides but if I'm going out for a longer ride I find it handy and intersting.
It's supposed to run 20 hrs on 2 AA batteries. It has a color display that will show your exact location on a color map. As you move along a road, the cursor (that's you) moves along that road on the map. You can look at the map as you ride and plan alternate routes by loooking at the road choices available and taking the one that leads to where you want to go.
It also has more functions than any cyclocomputer, including Avg Speed, speed, max speed, elevation, elevation gained (or lost), ascent or descent rate, sunrise/sunset times for your precise location, distance, distance to target, time, elapsed time, odometer, trip odometer, compass, estimated time of arrival at destination.
It also has a calculator, a calander, and an alarm clock. You can "type" in the address where you're going and it will direct you there, either via the fastest route, or over a route whose parameters you determine (no highways, no toll roads, no unpaved roads, for example). You can tell it to calculate your route for a bicycle, or a pedestrian, or a car, or a half dozen other methods of transportation.
It tells you where stores, restaurants, hospitals and parks are. It tells you the names of the road you're on, or the stream you just crossed over, or the park you just passed by.
You can "zoom" in so there is an incredible amount of detail on the map, or you can zoom out for the big picture. You can zoom way in (so the display shows an area of maybe 100' x 150'), or way out (the map displays North America), or anywhere in between.
These are just the facets of it that I've figured out so far.
I've got maybe 50% of it figured out. When I need to learn something about it, I find it is very easy to do, and very intuitive.
In short, these are amazing machines. When I tour I will have a GPS with me.