Rapcom gives some good general criteria.
More specifically I have used a Garmin Zumo 600 on the Rapide for thousands of miles and found it to be very good. The supplied bike mount fits easily onto the handle bars with plenty of adjustment to get the best angle for viewing when riding. It is quickly and easily removed from the mounting bracket to slip into your pocket at stops.
The unit has proved waterproof on some long and very wet rides.
The kit comes with a car mount too so that the unit can be used in a car.
I have used earphones plugged into the socket on the unit to receive the voice instructions and recently I have used the Bluetooth facility to wirelessly connect to my new Cardo Scala G4 intercom.
The Garmin maps seem quite good but would advise buying a "lifetime" update to keep abreast of new roads.
The internal battery lasts quite well but I usually use it plugged into a 12 volt socket on the bike.
If you want music on the move the Zumo can play mp3 files and the music is automatically muted when a navigation instruction is given.
It takes a micro SD card for additional storage of extra maps, music or jpg files so you can view pictures of your favourite bike etc.;-)
It is easy to put destinations into the unit using an address or postcode.
I use Garmin Mapsource software on my laptop to produce custom routes that I can then upload to the Zumo. (This is great for planning club runs where I want to avoid built up areas.)
Another popular waterproof motorcycle unit is by TomTom although I don't have personal experience of this.
The waterproof GPS devices unfortunately seem very expensive compared with units for "dry" car use.
Looking at getting a sat nav for the bike for touring and would be interested in what other members use and any tips.
Are specific motorcycle sat nav any better than car ones, they seem a lot more expensive.