I use a regular auto GPS, currently using a Nuvi 52LM on a Ram mount with VVMapping maps. The downside is that it's not waterproof but I put a plastic bag over it when there's any chance for it to get wet. The topographic information is not as good as a purpose-built off-road unit like the Montana. But the screen is more readable and best of all, they're cheap. I got the one I'm currently using for $35 on eBay. If it dies, I'll buy another one. I can buy a whole bunch of them for the $600 it would cost to get a new Montana. I'm planning to use it on the snowmobile this winter, it'll be interesting to see how it holds up there.
There are some good videos on youtube about using an auto GPS off-road.