This is one of the most comfortable etrier-style aiders to walk up because of the Biothane Step Stiffeners — that black plastic-like stuff that we think is just a fancy name for urethane. It keeps the steps open, sturdy underfoot, and keeps sides of the aiders from squeezing the sides of your foot. The Biothane also helps keep the steps open. The only downside to the Biothane is that it seems as though the steps are a little more likely to twist themselves inside out, especially the very bottom step, which seems to want to be twisted inside out more than it wants to be open. This is annoying at times but it happens with all aiders, just more with this one.
Of all the aiders we tested, this was the best for top-stepping for two reasons: 1) there are a good number of sub-steps and 2) the top sub-step is located higher than on any other aider we tested, meaning that this aider gives you the biggest reach in the highest step. In addition, the grab loop is big enough that you can slide your foot in it for "super top-stepping."
The same Biothane Step Stiffeners that keeps the steps open also mean the steps are a little more bulky for free climbing. Also, the Biothane seems to get more jammed in cracks below you than an aider without plastic-like step stiffeners. This is not a big issue if you are on a mostly aid big wall. But if you are doing a more free-climbing intensive wall such as Half Dome or Touchstone Wall, it will be annoying to free climb with these clipped to the side of your harness.
We have used these aiders a lot. They hold up great except for the most important spot: the stitching that holds the steps in place wears out after about 10 Grade VI walls. This causes the step to "zipper" open. This is not an issue for most climbers because most climber won't climb 10 Grade VI walls in their lifetime. Chris Mac contacted Metolius about this many years ago. They were responsive but said that not many other people had this problem. Chris says, "I think (and I am guessing here) the challenge for them is the ability to put bar tacks through the Biothane. They only have two bar tacks on the upper side that gets the most force while they have four bar tacks on the bottom, which doesn't have to deal with as much."
This is a great all around aider. It works great for super hard aid (Chris used them on Reticent Wall and most hard walls he has done).They also work well for less aid-intensive walls because they are not too heavy (although not super light either).
This is the best buy for aiders. You get a lot of very comfortable aider for $37. The Petzl WallStep is only slightly higher scoring but double the price.