I bought a Mitac Mio 268+ back in March and am really impressed by it. They've got new models out now which look even better. The 510 has replaced the 268+.
Make sure that whatever you get, it has got full postcode search facility, and the latest chipset - Sirfstar III - as this makes for faster satellite finding and route changing.
Mio are not a big seller in the UK so are realtively cheap as they are still trying to buy their way into the market. They are a big seller on the continent apparently. Oh, and there is a bike mount available as I've got one, although the unit is not waterproof. And when you select a route it asks what kind of vehicle you are driving - car, fast car, lorry, etc. But the best bit is if you select motorbike it will try to find the twistiest roads for you!
http://www.mio-tech.be/en/index.htm to buy I used these guys. Very good service.
http://www.superetrader.co.uk/
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