Silverblue, to answer your first question as to the selling rates, I cant say i know as I dont carry the petzl biners. (havent bought into petzl yet, lack of demand). The biners I do have are DMM/Wales and I would consider them a tri-act version, if you will. Being these the only line i have, they sell the best. I have been using dmm/wales for yrs and can open them one handed w/ ease. Usually once i show someone how i do it, it makes it sooo much easier. I cant say either ive ever lost a sale simply because i didnt have the petzls, although things like that are usually never said.
As for complaints, a buddy of mine bought a couple of ball locks a while back and recently had the ball fall out of his rendering it basically useless. His only other real complaint was the difficulty opening it w/ heavy gloves on.
I have tried the ball lox just in play and didnt really care for them. I too found it hard to open. However i will admit to not really giving them a 'Fair' shake by using it on a long term case.
So, would i carry them - Yes. If demand existed for me.