First determine if you really need a GPS.
I actually have two,a Garmin 176 and a Garmin Street pilot 3.Even so, there are lots of times that I just use my plain old map.
If you decide a GPS is for you,or you like "toys" on your bike,then the most common
ones in use are Garmins,particularly the Streetpilot 3,the 176/276/376 series and the 2610 and now 2700 series.
Look for a model that doesn't depend on external power all the time,that way you can route plan off the bike.
Look for a model that is reasonably weatherproof.
Look for a model with expansion cards and one that can upload routes created on a PC,none are readily Mac compatible by the way.Don't get a microdrive based model such as the Garmin 2620.
Look for a decent size,easily viewable screen,remember that anytime you look at the GPS your eyes are off traffic!!! You want to be able to glance at the GPS and get the info you need in the briefest amount of time.
Look for good adjustable backlighting and easy readability in sunlight.
All of the above models fill the above requirements to a great extent and have found favor with MC riders over the past several years.Of course new and better models are always coming out.
No suction cup mounting will work on the bike,TOO much vibration,the cup will come loose.