I've been using my Montana since I bought the Tenere.
I love it, but...
Even with City Navigator maps, it can sometimes mislead you. In my instance, I was trying to follow backroads to my destination, whereas the Montana kept trying to get me to backtrack, go back to the interstate highway, then follow it to my destination, even though my destination was less than 20 miles form me, it insisted I backtrack and go around the long way via the interstate.
Now, I don't think this is an issue with the Garmin, but rather I suspect I had one or more of my settings selected incorrectly.
Which brings us back to a point made earlier - the learning curve. To just click through all the options on the Montana (I have the 650t), will take you over 30 minutes - that's just to click and explore. Many things are intuitive, while many are simply arcane and you must "learn the Montana way" or remain flumuxed.
I've found it super easy to add maps to, then find them - though there is a distinct difference between routes and tracks, so choose your option carefully. Each has its own use, though either will do in a pinch.
I found basecamp somewhat easy to use when making my own routes, but a couple false starts had me using the tried and true "if that didn't work, then maybe this will work" method. So like the Montana, Basecamp has a learning curve.