A new convert to the Stack here. I'm a bit different than most of the people on this thread as I am in my mid-60s and most definitely not swinging the Stack or my driver in the 100s LOL. Due to a series of unrelated but simultaneous issues, I was unable to touch my golf clubs for the last 2 years after having played regularly for most of my life. While getting out on the golf course is still not realistic at the moment, I am now able to practice again and am committed to rebuilding my game even if only at a practice facility for now. I started hitting balls and working on my short game in early January, and acquired the Stack in mid-February. My realistic goal was to get my driver speed back up to around 95 mph which is about 7-8 mph more than where I found myself in January, and to pick up about 1 club in my irons.
Anyway, my first Stack Foundation session was a bit of an anomaly as it was well above my baseline results. I returned to my 195g baseline in session 2 and have made steady progress each session thereafter and today in session 6 I finally got over the top of those session #1 numbers and up 6mph from baseline. That Stack gain appears to be translating to about +1mph gain in my short irons, +1.5mph in my mid-irons and TBD in my driver (after 2 years not swinging a club, I'm still working to get comfortable and consistent with my driver swing).
I find my biggest challenge has been the stress that practicing and using the Stack has had on my core muscles. I generally have been hitting balls on Monday-Wednesday-Friday since mid-January. I have experimented with incorporating my Stack training into those practice sessions or alternatively doing the Stack training on the off days. Either way, I've been struggling to recover between practice sessions and, while I'm not injured, I'm having to work hard to get warmed up on those hitting days. I think I may try taking the 2nd day of rest between Stack sessions and see if that helps (although it will be a challenge for me to show that level of patience).