It's all about the steps: beefy 1" webbing is covered with beefy 1.75" webbing to create the most comfortable steps out there. The beefy plastic spreader bar up top does two things: 1) It keeps the steps open so you can get your foot in and 2) keeps the aider from squeezing the sides of your foot. The steps are close enough together that, when selecting gear, you can rest on two feet rather than having one foot flagged in space as with most sing Etriers. Then when you reach up to place a piece you stand just on your one top foot. This is important, especially for people who are moving from using four aiders to just two. The construction is bomber. Chris Mac says he has put more than 15 big wall ascents in his and has yet to wear a pair out.
One extra feature is the elastic strap under a few of the steps that keeps your foot from coming out of the aider. Some people, especially beginners, might appreciate this. However, we don't use this feature because it takes more time and makes it annoying when you want to quickly step out of the aiders while cleaning. Most importantly, over time you will get good enough at ascending that your feet won't come out of the aiders.
When Chris Mac teaches aid climbing, he finds that most people have a much easier time with this aider because of the intuitive ladder style design and generously sized steps. He now only teaches with ladder style aiders.
The downside to this aider is the weight and bulk that are the tradeoff for the aider being so comfortable and durable. For a free climbing-intensive aid wall such as The Nose, this aider is bulky and heavy when clipped to the side of your harness while making free moves. Also, cracks like to eat these aider steps. If you're cleaning with these aiders, you can definitely feel that they are heavier, especially when pushing up the top ascender. However, at the belay you will be happy to stand around in such comfy steps.
Chris Mac recommends this aider when teaching people to aid climb or when he is doing more aid-intensive big walls (as opposed to walls that have a lot of free climbing such as El Capitan's Nose). He has used the 6 step length but very tall people or people doing really hard aid might consider the 7 step length. Chris has considered chopping off a step to make a 5 step aider for more free intensive routes like The Nose where normally he would use a standard Etrier style.
Chris says, "I have used these a lot after Ammon McNeely showed them to me then gave me a pair (thanks Ammon!). Up until that point, I was a diehard user of the standard aider style or "etrier" style. But after using these for one wall I was a convert to the Yates Big Wall Ladder. I have used them for every aid-instensive big wall I have done in the last five years."
This aider is $10-15 more than most other aiders except for the Petzl. We think the price premium is worth it, but if you are on a strict budget or are not going to do many walls, you might consider a less expensive aider Such as the Metolius 5 Step Aider or Fish Smart Aider.