I mtbike with my Edge 305 mounted on my stem. Not a problem but I'm not super aggressive. Some table top jumps with a bit of air when I'm feeling frisky, but that's it. I also have a Forerunner 305. Neither is going to get you un-lost. These are not mapping GPS but recording units, as in they record the coordinates you've ridden. If you can match up your coordinates with a good map, they can help. But if you are standing at the intersection of 5 unmarked forest service roads/trails and the map only shows 3, and you want to map a route to another set of coordinates, good luck.

If you want a true mapping GPS with topo map updates, you need to look at something like a DeLorme PN-40. From what I've read, DeLorme does a much better job with topo maps than the Garmin 705. And the 705 is not built for trail riding.
http://shop.delorme.com/OA_HTML/DELi...?section=10461

If all you care about is recording where you've been with a bit of idea where you are, the Edge 305 works fine. But don't expect it to get you home if you're lost. And carry a compass anyway. Don't depend on an electronic gadget out in the wild.