Of course a must do! A great backpacking trip if you camp in little Yosemite Valley at the base of Half Dome. If you're not a climber then those cables are the closest thing you can get to a climbing experience in the valley!
The hike up the cables on Half Domed scared the hell out of some of my friends; not because they thought they would fall but because they thought somebody above them would!
Forwarned, I brought leather gloves, a climbing harness, two long slings, and three carabiners. I kept myself clipped into the cables at all times (al la via ferrata)so if somebody above fell and hit me, I wouldn't go all the way to the bottom.
Also, don't head up the Dome if it looks like a T-storm is on the way.