I use a Nuvi 205W. It's inexpensive (think Walmart has them for around $120 or so now), and as V65 says, baggies work just fine in the rain. I would recommend a Nuvi model with bluetooth so you can use a helmet set (Scala, etc.) to hear the spoken directions or use the unit in concert with your cell phone (answering calls, not dialing while moving....figured I would put that disclaimer in here for all the safety police that will probably comment). I probably have 1000 miles on it so far and I swap it back and forth from my bike to my truck. So far so good. The touch screen on the Nuvi also works well with all of my gloves.
I use industrial grade Velcro (bought at home depot...found out about this stuff from a paraplegic friend of mine who depends on Velcro attachments) to attach it to a home-made mounting plate attached to a ram ball and mount. Also use this heavy duty Velcro to mount my Escort 9500i in exactly the same way.
Back to the Nuvi....it will be interesting to see how it does hold up over time, but its doing fine now, and plenty visible in the daylight. Battery time averages around 3 to 4 hours, depending on where the brightness level is set. I am running a dedicated B+ wire soon, so battery limitations will be history.
I can buy 4 of these for what you would pay for a single Zumo 550. Not detracting from the Zumo units, but the Nuvi serves my purposes well, and at this point I consider it a practical GPS solution for the average rider (me). Good fortune in your decision!