I'll give my experience with Esther's program in general, including the sit cushion. Sorry for the wall-of-text.
tl;dr = the cushion and Esther's program was a part of me finally being pain free after 20 years. How big a part? I don't know, but I have to recommend it.
About a year ago I came across Primal/Paleo, CrossFit and Gokhale all at roughly the same time. Not surprising really since I learned about them from roughly the same sources. I was 43, in OK but not great shape, not overweight but a pretty shitty eater (SAD). Prior to this I had suffered from lower back pain off and on for about 20 years. Nothing really helped much (chiro, acupuncture, Egoscue).
I'm fortunate to live pretty close to Palo Alto so I sucked it up and dropped the money for Esther's 6-week class. Pretty desperate for any relief so I figured it was worth it. I bought the book and 2 cushions (one for my car, liked it so I bought one for my office chair).
I put a lot of effort into following everything taught in the class and the book. A lot of time it comes down to remembering to do stuff, so I would often find myself slouching, walking poorly, etc. But all-in-all I think I did fairly well.
Oddly, though, during the course of the 6-week class itself I feel like things with my back got worse. Now, I think some of this may have to do with making a lot of changes to my posture, etc. But also, like I said, I was just starting to do crossfit. If you're an x-fitter you know that you do a shit load of squats, deadlifts, wallballs, cleans, etc. All of these were very difficult for me initially (I could basically deadlift a bar, not much more) ... like I said, my back felt worse a lot of the time.
But, I felt like I was on to something and stuck with it. And then a miracle ... somehow, somewhere (maybe 6 months) along the way I really did start getting stronger and my back pain basically disappeared. Was it crossfit? Gokhale? Both? I don't know and I don't really care. What I do know is that I'm pretty much pain free for 6 months ... something I never could have imagined.
So, my impressions of the cushion. I really like it. It's much better than a rolled up towel if for no other reason than ease of use. Having it on my office chair and car seat almost guaranteed lots of stretch-sitting time. It's a perfect, physical reminder to sit yourself correctly.
Durability-wise the one I had in my car took quite a beating, a couple of the rubber nubs came apart at the seams (though the one in my office had no such problem). By the time it fell apart though I wasn't needing it any longer so I didn't really try to get Gokhale to replace it. They might very well have ... I just don't know.
I really liked stretch-sitting a lot so I'd say the $50 bucks was worth it for me (my company paid for the office one). Whether or not it's worth it to you really depends on how bad you feel. Like I said I don't really know if the Gokhale method was the key to my success ... tbh I kind of think it was more down to the cross-core strength I gained from Crossfit. But to be honest I would have happily paid 10 x the $50 if you'd told me I was going to be pain free, whether the cushion had anything to do with it or not.
Best of luck.
P.S. Now if I can just kick my goddamn sugar addiction and get back to eating paleo. :)