I've used it pretty much exclusively for years, I'd be very interested to here what prices they are charging you? My place doesn't sell an s1 Rapid, only the trade flexi Rapid like the one in the photo, it's a c2. The semi rapid and the standard setaflex are s1. I've always loved it but recently I've used a few other products again and it's highlighted to me how far tilemaster doesn't seem to go!
I use it my self usually but the job I'm on at the moment I've bin using kerakoll H40 no limit and I can't fault it its very similar to tilemaster lightweight but it sticks literally anything to anything I paid about £18 per 25kg and it goes really far I'd say about 6 or 7m per bag with a 12mm notch and back skim