When i'm tradding, I keep two of these in the zipper pouch on the side of my chalk bag with some rap rings and a blade. Makes a nice little "get out of trouble" kit. Use them with caution though, if you let it slide on your rope rather than pressing it in with your thumb, as the directions show, it'll shred it. And the hard steel is pretty rough on some biners. Had to use them once so far to get my second out of some trouble. Direct belay with a reverso + 1 tibloc = Z pulley reduction. If you're creative, you can ascend with just one and a reverso/ATC-G too. Good to have, but really hope to never use.

wow! INGENIUS! quite possibly the coolest yet primative piece in my rack. cuts prusik time in half!
great for hauling, or hanging around while your taking pictures!

In an unexpected emergency situation, where you do not have your assenders, these little buggas are a life saver. I wouldn't use them unless I had to due to the rope issues, but I always have them on my rack. Small, light and easy to set up.

need to practice rigging and using it before you have to on a rainy, cold night in a state of panic. carried it until I bought the ropeman markII ascender

If you don't have the cash for a full sized ascender or just don't want to carry the extra weight, this could be nice to have in a pinch. However, it's just not good for regular, everyday use.