I bought a Nuvi 550 earlier this year. I'd purchased a K75S that was located in California and I planned to fly out there and ride it back to Ohio. The seller informed me that the K75S speedo stopped working just before I went out to get the bike.
So, I got the Nuvi 550 mainly to serve as a default speedo. Plus I hoped to avoid the superhighways (e.g., Interstates) and ride some backroads.
The Nuvi 550 was my first experience with a GPS on a bike (I've used one connected to a computer in a car before with me being the passenger using the GPS). I used the RAM mount for the NUVI. I obtained a mount from Cycle Gadgets. Subsequently I've seen the mount from RAM. In my opinion the mount from Cycle Gadgets is superior.
Why? Well there is an easier to deploy snap-type top fastener that securely holds the GPS. This looks like it will last longer than the solid plastic mount sold by RAM. The RAM mount is slightly less expensive.
I decided to use a BMW plug on the power supply. When I'd stop for a meal or for the evening it was very easy to disconnect the GPS and power supply. The bulk of the cables for the power supply were kept in my tank bag. It was very easy to move / remove the tankbag to add fuel or when going in somewhere (dinner, hotel, etc.)
Happily my BMW K75S speedo started working again in Colorado (after going thru California, Nevada, and Utah). The GPS confirmed that the BMW speedo adds 5 mph to the actual speed.
Since I stayed within the US I could have gotten the Nuvi 500 rather than the 550. At the time I purchased the Nuvi 550 the lowest price that I could find was $225. I thought that the advertised price of $150 was quite good for a waterproof GPS suitable for motorcycle use.
I've come to really appreciate features on the Garmin Nuvi 550 such as having a large database with hotels, restaurants, gas stations, museums, partk, etc. conveniently placed. One can select a point-of-interest or enter an address and let the GPS provide directions. If a turn is missed the GPS quickly recomputes a new route.
In missing a few turns in my cross-country trip I was able to see some interesting countryside and still arrive at my intended destination. Some of the side trips were worth the price of the GPS!!