I did not use one off road but I did use it for an on road trip. It worked fine. But if you want to use it as your primary GPS then I think it is a mistake.

First the GPS is not as sensitive as a dedicated GPS so you might not get a fix where otherwise you would on a conventional GPS.
You battery life will not be all that great. You will be very lucky to last the day.
It is just too bulky to have out all the time unless you can come up with some kind of mount, but then you subject it to possible damage.

It is a great supplement to a normal GPS as it makes route planning so much easier. But if you want to use it for turn by turn then not so great.