After climbing with cables up and cables down I will only ever do it with them down from now on. There are almost no crowds (we didn't see anyone on the trail until we got to the sub dome) and there were only a hand full of people at the cables. Most were campers from Little Yosemite campground.
I was, however, very surprised at the "safety" rigs that everyone else was using. They had proper climbing harnesses but were using ropes with carabiners to clip onto the cables. In my opinion, all that will do is keep your body close to the cable while you fall to the next anchor point. At which time, you will likely suffer injury from the stop, or worse, the biner fails from lateral load on the anchor and you continue to the bottom and experience sudden deceleration injury (splat) on the granite below. I have good confidence in my physical abilities but I don't like putting my life literally in my hands (grip to the cables). A wife, two kids and a mortgage makes one think a little harder about that.
The rig I use, which works very well, is the following:
1. About 6m of a thick climbing rope tied into a swiss seat (there are many videos online showing how to tie one) - I like to wrap the rope an extra time around the waist for comfort, the thicker rope is for comfort too. You can use a climbing harness but the rope is cheaper (~ $3.50/meter) and perfectly adequate for this purpose (I am a cheapskate).
2. Two prussic loops of about 2.5 - 3 meters each ( thinner climbing rope ~ $1.25/meter at your climbing store - USE CLIMBING ROPE NOT HOME DEPOT ROPE). (instructions for tying a prussic loop using double fisherman's bend at: http://www.animatedknots.com/doublefishermans/index.php
also look here to learn to tie the prussic knot on the cable: http://www.animatedknots.com/prusik/index.php
3. Two large locking carabiners (~ $8 - $10 each). You can do it with one (I do) but two will allow you to dedicate a prussic loop to each biner so you are not removing one loop from the same open biner that is holding your active loop (lifeline).
4. Gloves - latex palmed gloves ($3.00 @ Home Depot) for the ascent, leather gloves for decent (optional) - the latex tends to get grabbed by the prussic knot on the way down as you slide it. You want gloves because there are some wicked "spurs" of wire at some spots that will do a number on your hands if you slide over them ungloved.
Tie your first prussic loop to the cables and proceed up, keeping the knot loose and ahead of you.
Test the knot, the rope, and your confidence early on the cables. Plant your feet on the rock and lean back letting the rope hold you. This rest position will save your arms and calves as you proceed up.
Keep your 2nd loop over your head and shoulder so it doesn't drag and become a trip hazard. As you get to an anchor point tie your 2nd loop above it with a prussic, clip it into your 2nd biner (or on your one binder and carefully pull the first loop BETWEEN the 2nd loop and remove it from the biner). Remove the 1st loop from the cable and head on up. At no time were you detached from the cables so your momma will be happy.
With this method it is almost impossible to fall. I took two complete beginners up the other day and they were never afraid with this rig and were able to enjoy the view and experience vs. being concerned if they could do it. There were others who had the harness/biner rigs who got part way up and "froze" when they realized how bad things could get and came back down.
Coming down is easy, you can back down as you slide the knot along the cable or (my preference) you can face down and slide the knot along with one hand behind you - more fun but takes a little more "guts".
Bottom line, and I hesitate to post this for fear of more crowds when the cables are down, it is very doable but do it smart. I enjoy risks, but calculated risks with the right equipment.
Here is a picture of the rig: