A view from the bottom of the cables, straight up the side of Half Dome.  JD is about 25ft up and has about 700ft more to go.  As you can see, there are cross boards about every 20ft.  This is a good thing since the rock is inherently slippery, made more so by the polishing effect of many thousands of shoes.

For some, usually younger, folks the exertion factor is not too bad.  For the aging but active (me) it is a test of aerobic endurance.  I strongly recommend taking your time and enjoying the unparalleled view on the way up no matter what your fitness level.  However, pictures from the cables should only be managed by the young and supremely confident.  Losing one's balance here has fatal consequences.