Batteries: using your blackberry like a GPS, keeping the screen on all the time, and continuously downloading maps, runs the battery down. And you can't just slap in 4 fresh AA's.
Weather resistance: Blackberries are not tolerant of water and are not designed to outdoor gear water resistance specs. Water intrusion voids your warranty.
Display: on the brightest settings, it's difficult to read in direct sunlight.
Signal: if you hike out of reach of cell phone coverage, your GPS will still work and give you lat/lon info, but you won't have maps. There are only a couple of GPS apps that let you store maps on microSD cards.
This happened to me two weeks ago. I had Sprint's GPS service working great on the main road, but as soon as I left the road the GPS stopped working.
For the record, I had no cell coverage, but was able to use the GPS somehow. I know it shouldn't have worked, but I was able to use GPS and not make calls for about a period of a half hour.
