ronen sarig wrote an answer about GoPro Chest Mount Harness on December 22, 2010
first of all, its a different view. expect to see more bars and arms, and if you're sitting down, knees pumping the pedals while wearing the chest harness.
second main difference: i've noticed that when i do sports of any sort, my head is remarkably still. the helmet mount gives a super smooth video. (while your torso may bounce around, your neck absorbs the shock, and your head stays still.)
also, if you don't tighten the chest harness down quite a bit, it will bounce when you hit big jumps. but even when tight, you don't really notice it there...