Years ago I used to use a Nuvi 360 on the bike, and if you just want to let it choose your route they're OK - as long as it doesn't rain.
So I spent several weeks on Mapsource planning a trip across France only to find the Nuvi doesn't work with planned routes.
I managed to sort of follow the route I wanted by using favourites as destinations, which wasn't easy or brilliant.
Then I hit torrential rain. So I managed to keep the Nuvi dry with a plastic bag, but the bag kept misting up, and trying to clear it changed screens and zoom level, then I'm guessing the Nuvi thought the clip securing the bag was the traffic info antenna so every five minutes I had to cancel a warning telling me I'd not subscribed, please visit garmin.com. Then I entered the wrong favourite as the next destination and ended up on the peage headed in the wrong direction trying to figure out what went wrong. All this with trying to clear my visor and see where I was going...
So the torrential rain turned biblical and everyone - cars, trucks, everything on the peage turned into a service station as the standing water on the carriageway made it impossible to see where the road was, proper scary stuff. So I followed a car into the services where the only shelter, over the filling station part, was jam packed with cars. So I had to sit it out in rain so hard it was painful, despite my bike kit, and figure out where I was supposed to be heading.
I ended up letting the Nuvi take me direct to the ferry port, which involved another 20km in the wrong direction. The rain eased off from painful, but didn't stop all the way... I remember the girl in the peage toll booth handling my soggy notes for payment as if they were infectious

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First thing I did when I got home was order a Zumo!