The 12.9” is great for viewing and editing photo’s and 4K video.
I recorded footage of several stage performances, using a Sony A7R II and loxia 21mm, where you can just use maximum zoom on parts of the stage as if that was the main recording. Scroll at leasure to left and right, hugely impressive! On the 10.5” it doesn’t quite look as cool. Nor do photo’s.
I use a 12.9” but I don’t draw. I do write a lot of notes and quick sketch diagrams (napkin stuff, not art) for work projects. It’s just a lot less of a headache than carrying multiple spiral notebooks and trying to find my notes/diagrams.
I'm an engineer who doesn't draw. I got the 12.9" because my hands are big enough to hold it in landscape and reach the YGHB keys with my thumbs. It was almost possible on the 2017 12.9" and this most annoying about it has been fixed by removing most of the bezel.
I am an engineering director and I use the 12.9" iPad pro to review documents, create and edit office documents (PPTs/excel/etc), email, skype meetings, etc.
Office 365 is great on the new iPad pro. This week I had PowerPoint on one pane (70%), sharepoint on the other pane (30%), and mail as a slide over. I was able to create all the speaker notes for 5 different training modules we are deploying, which included copying pictures and text from the sharepoint site and my own speaker notes from email into the files.
OneDrive -> PPT -> OneDrive... no file system needed.
Laying out PPT slides with my finger is so much more satisfying than using a mouse to move graphics around!
12.9 user here who doesn’t draw. I do however like taking handwritten notes for myself and sketch out some ideas every now and then.
Buy the device if you think it gives you a good experience, you’ll be the best judge of course.
I also enjoy the big screen for reading, split-view operation and photos of course.