I always used a foot stool, even before I started playing CG. Yes, I always used a footstool even with my steel string acoustic and my electrics. I know, unusual
So I never had a problem with a foot stool...until I started with CG. I got pains in the mid-right side of my back sometimes. I bought an Ergoplay, the "improved" one, which is more adjustable. I intended to use both; one day the Ergoplay, the other day a foot stool. But I just can't use it. I've tried, really. Hours and hours of fiddling and testing. The problem I have is that it simply pushes the guitar out towards my right leg too much. The cheaper, less adjustable model would probably have been better, judging from the pics.
I found out something else though. The pain in the back comes with tension. When I'm learning new pieces or working on difficult parts, my back aches. When I'm nice and relaxed, playing "easier" stuff, then I get no back pain. The pain comes pretty quickly too, so it has become quite a good indicator of tension. So, now, whenever I feel the pain sneaking up I sway around a bit with my upper body (while playing), consciously relax, and the pain goes away.
I'm 100% convinced that an apparatus such as the Ergoplay is much more ergonomically sound than a foot stool. But I think the foot stool is not as bad as people make it out to be. I think the major contributor to the back pain with a foot stool is tension coupled with a foot stool
, not the foot stool itself.