This is the best aider to free climb with. Not only is it lightweight, but it also fits into its own handy little bag which means there is very little bulk and you don't have to worry about it snagging on stuff. This aider does have a well located top step. The little stuff bag does get in the way a little, but is not a big deal. But because the aider material is so light, you often have to use you hand to get your foot into the top step.
One unique feature is the Petzl String which is an not intuitive name for the piece of rubber that you commonly find at the end of Petzl Spirit Express quickdraw. It protects the runner from wearing at the carabiner contact point. It also just gives the aider better structure. It doesn't flop around.
Because it uses skinny and soft 3/4 inch webbing, this aider is not comfortable to stand in. It works great on a route like The Nose on the pitches with mostly free climbing. However, when you get to a pitch like the Great Roof, the downside of such a skinny aider quickly becomes apparent. The bottom and sides of your foot will be aching anytime you stand in them for longer than a few minutes at a time. Because the material is so light weight and skinny, it's not very easy to walk up these aiders. The steps don't stay open well compared to the Metolius Alpine Aider. But the Metolius Alpine Aider also doesn't compress small enough to get in a little bag. So its a trade off. Also, the steps turn themselves inside out more than most aiders which takes extra time to fix. Lastly, there is no grab loop which is a downside. This is not the most durable aider. The steps themselves have held up well. What is a bigger issue is the bag which is relatively fragile. While I have not personally ripped it, i have a friend who has after only a few walls (it was easy to re-stitch to fix). So keep in mind you might need to handle them a little more carefully than most aiders. Because the aiders are a little stretchy, it makes it a little more difficult to bounce test.
This excels at wall routes were there is 90% free climbing with a couple aid pitches. For example, I would take this on the Regular Route on half dome because I want to travel light on the big approach and there are really only a few hundred feet of aid.
At $45 and for such a limited application, this is a pretty expensive aider.