I currently use a Garmin Montana, previously used the Oregon. I have the same issue that Jim mentioned with very limited cell service the places I go in CO. Yes, I know I could download the maps into my mobile, I just prefer to have a dedicated and waterproof GPS device so I can leave my mobile with better battery life throughout the day if I do end up needing it. For the Garmin Montana, Amps makes a Rugged Mount powered cradle that combined with the RAM handlebar mount, is as good or better a combo then anything available for GPS or phone. My Amps mount is directly wired to my battery so my GPS does not turn off when I turn off my Grizzly.
A big + for me going with the dedicated GPS is the ease of ability to load in Funtrek tracks and now, CO has an incredibly useful, online, state trail mapping ability. I can go on the CO site, search for OHV trails under 50" and link together a trail system. Save it and then download the tracks and upload the tracks into my Garmin. It is pretty easy to do and came in very handy for a massive trail system I had never been on prior to Sept this year. More states need to make similar websites available.