I'm kind of looking myself, and still debating what kind to get---- an off-road/hiking version like in the first post, or a Nuvi similar to the 550 listed above (or just buy a Ram mount for my current street Nuvi and put a plastic baggie over it if rain threatens). I'm kind of leaning towards the Nuvi 500: same as the 550, except it also has 1:100,000 scale topographic maps pre-loaded on it but doesn't include street maps of Canada. From what I've read so far, the small hiking-style GPS receivers are better in the off-the-beaten-path environment, but may not include street maps and are lacking in terms of big screens, voice commands for turns, etc.... that we all expect in a car/truck GPS. The Nuvi 500 is a crossover-type device that attempts to be a little of both. Here is a link to one vendor and some of the accessories available for mounting, power, etc...: http://www.gpsonsale...CFQEhDQod1Q-a4w
You will have to do some Googling and decide which features and usages are more important to you--- or have a dedicated GPS for each purpose. I am finding many different forums and sources on the internet, including Amazon and these: http://www.gpsfiledepot.com/
Be aware that it can drive you crazy trying to determine what is hype vs what is truth on the capabilites of the different devices. For example, the Nuvi 500 descriptions from Garmin makes it seem like it will do EVERYTHING right out of the box, but in reading the different forums you will discover that some usages may require the purchase of other, more detailed topo maps (or special marine maps), it will not speak street names unless you download a free update and free voices from Garmin, etc..... Garmin is not alone in this, so like anything you will have to do some research to see if a particular device will meet your expectations.
The whole topic of mapsets currently has my head spinning: 1:100,000 scale vs 1:24,000 scale (the smaller number gives more detailed/close-up information but may require you to have smaller areas of the country loaded into memory at a given time), maps from the USGS (US Geological Service) vs from the various map-making companies like Navtec or DeLorme, free maps, custom maps, satellite maps, and many more. Everything kind of depends on what activity you plan to do with the information.
I'm currently trying to force myself to be realistic about where I will be riding and the kind of riding I'll be doing. The chances of my fat, old butt leaving the Midwest to off-road or hike in the mountains or desert is pretty slim, while the chance of me putting around the backroads of southern Wisconsin are much greater (and maybe a jaunt on forest service roads in the Upper Peninsula of Michigan). That is what has me currently thinking about a cross-over type of device like the Nuvi 500: it may give me more of what I'd like on the rural roads I'd normally be on, the included 1:100,000 scale topo maps may be all I need to follow a main forest service road, and I could buy a download or pre-loaded memory chip (or possibly a free download ) of a more detailed map segment for a particular area such as the Upper Peninsula.
Enjoy the hunt for that perfect device, and let us know what you decide!