I bought a used b/w Garmin Streetpilot to use in my Mercedes motorhome when I spent the summer in the UK this year. I bought a 64k chip for it. I was able to get just about the whole of the UK on that chip. It was absolutely indispensable for my journey. I don't have any experience with other GPS's but the Garmin was always spot one and reliable. I could also put it in my pack and use it while hiking. The newer color versions will give voice instructions. I did not need this since I was travelling with my wife and she could view the screen. Travelling alone I could see the usefulness of voice instructions.
I learned of the value and precision of the Garmin shortly after setting out from Exeter where I stored the motorhome. I had travelled for about half and hour and not paid attention to the GPS. I stopped for fuel and picked up the GPS to see if it was at all accurate. The screen showed me in the BP fuel station! What more could you want?

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13