Having just purchased my 10.5 Pro I am in two minds as to whether I should purchase the leather sleeve.
On one hand I am a big fan of Apple's own accessories as they fit perfectly and look great and I can certainly see the benefit of the leather sleeve given Apple don't offer a back cover for the 10.5 and I do worry about the back getting scratched in my bag and on my travels, coupled with the Apple Pencil holder.
On the other hand, £129 (UK) for what is in effect just a sleeve albeit a beautiful looking one is a lot of money to justify when there are sleeves out there that can do the same job for a fraction of the price.
I am still undecided at this moment in time and intend to go to my local Apple Store and try one out for real.