I have used an inexpensive Garmin Nuvi for years and it works great.
It plugs into the lighter socket but buy yourself a good handle bar mount.
A few years back, I had mine suctioned onto the tach face (dont really need a tach on the harly anyway) and it managed to escape while I was doing about 40 mph over a rough road. Found it lying in the ditch and other than a few scuffs and scrapes, it still works good as new to this day.
As already suggested, keep a zip lock baggie handy for rain but they are pretty tough little units.
You could buy 6 of the inexpensive units for the price of one of those fancy "motorcycle" units.