I bought myself a Garmin Zumo 550 GPS in December, and I'd like to share my experiences.
This is a GREAT GPS especially as its nearly waterproof which I beleive is the only one on the market, the 550 comes complete with a bike AND car fitting kit, 2x power leads, bike connects directly to the battery, car is cigarette lighter type, NOTE the 450 and 500 comes with the bike ONLY option, also you get a computer program called mapsource so you can upload and down load places, settings, and tracks of where you have been.
The internal maps are hard wired into the Zumo so if you buy one from abroad you will also have to purchase UK maps and mapsource for the UK, so this doesn't make it a cheap option.
When in the car the holder has an internal speaker, for bike use you will need either a blue tooth headset (scala rider about £99), or headphones with an extension lead or an autocom etc kit (nothing for bike use is supplied). The Zumo also connects to your mobile phone via bluetooth so you can make or answer calls while on the move. The internal SD card (not supplied) allows you to listen to MP3 tracks while riding. I have also purchased for £20 per year a subscription to the speed camera database and this lets me know when they are coming up (this was my main reason for buying one)
In March the unit packed up (normally they are really reliable) I sent it back to Garmin who without hesitation sent me out a complete new set, fittings and all.
If your looking at fitting this to a sports bike (I have a GSXR 750 K2) you will have to make your own bracket or buy one (about £35) as these were made to fit to either the clutch or front brake lever of bikes with no fairing.
I also bought a GTX12 traffic receiver for car use and this lets you know of traffic hotspots and will detour around them, at the same time I got an external aerial thinking that in London it wouldn't work well, how wrong was I, the aerial is still in the bag as the Zumo has excellent reception.
I know its a lot of money but you are getting 2 for the price of one and if anything happened to mine I WOULD buy another.
Nearly 1 year on, and the Zumo is still going strong in the car and bike, and I also like to think that its saved my license on more than one occasion
UPDATE - UPDATE
8 years on, and my ZUMO just keeps on working, its covered tens of thousands of miles both in the car and on the bike and its never missed a beat - it could do with a map update now as theres a few roads that have since been built but if you know your way threw the area it soon works out where it is and your off again.
I'm SO glad I bit the bullet and paid the money for this - its great