I use one. It was produced by my saddler as an alternative not long after she uttered the words 'bespoke saddle' (I think not!) for my then 5 yo... Cost about £100 but it's certainly bought me time to get the youngster going in a saddle that fits me if not the horse, yet. Horse seems to have no complaints and goes well enough in it. I much prefer the idea of riding on an air bag to stuffing layers of prolite underneath. It took a little bit of fiddling around to set up but I've just repumped it a bit once in the last 12 months and don't know that it even needed that.
I bought one to correct my boy's asymmetry. It leveled him up and it worked pretty quickly. Although I aren't currently using it now he's level, it is a piece of kit I will never get rid of! A friend on the yard is currently using it to level her mare up.
If you were thinking of using it for stopping your saddle slipping forward I'm not sure if it would work for that?
You could buy a car cleaning mitt from Halfords (£3ish) and stitch to your numnah where he/she is lacking muscle. That was a tip from a physio and it worked for me. Once the muscle developed again I took it off.