No! I quit using mine, for that very reason.
I used mine to hold me in place, while hunting from my platform, until last fall, when I noticed it creeping an inch or two. That creeping or slippage isn't acceptable....period. If I were hanging off to the side and attempting to take a shot when it started slipping, that could cause me to miss out on the shot opportunity or worse....cause me to lose my footing and have an accident of some sort.
I use a Petroacan Hitch for ascending and switch over to a carabiner with a Munter or Super Munter for rappelling. Both are very simple to use, are way less expensive and are 100% reliable.

This is a 5 wrap Petroacan Hitch (tied the opposite of the Michoacan Hitch), using a 23" eye to eye prusik cord, from Ape Canyon Outfitters.
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This is the Munter Hitch with a large pear-shaped Petzl Attache carabiner. The larger size and shape handles extra wraps easier and has a larger surface area for less wear on the rope.
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