I've had a petzl grigri for ten + years. I first used in for mountaineering applications, then rock climbing, now tree work. It is very useful, easy to use, tough, etc... I have trusted my life to it many times, and has never given me a scare. That being said it only works with certain diameter ropes, usually standard climbing line around 10/11 mil, you can use thinner or thicker ropes but there are consequences for doing so. Thin ropes will be slow to engage the cam, you can fall several feet or more before it engages. Thicker ropes and it just gums up the cam, have to force feet it etc... For the correct size rope, for both climbing and rappelling, it is absolutely awesome, and will always be part of my saddle/harness rack. One issue with it that should be kept in mind is that it can be gummed up by a sappy rope. I don't work in a lot of sappy trees but I know they're common in certain regions.