I bought the 12.9 the day it came out, to read lot's of PDF's for my law study that I started last year. It's brilliant for that, legal sizes and A4 sized scanned documents from the 1900's-1980 aren't always that good readable and certainly not digital beside scanned pdf's. But as it happens I could use the big iPad for much more. I'm a consultant for SME's and I always disliked the screen of a laptop standing in between the customer and me. I like to write with a pen, yet I'm not going to write notes during a meeting only to type them out again afterwards. Here the big ipad Pro shine's. It's so much like a A4 paper-notepad, but with the MySync Nebo app it translates my writing to text. So the iPad Pro became my main work machine as well.
But al my law-study-books, readers and assignments are digital as well and I found that summarizing them by writing notes is the best way to learn what is in them. So I bought the iPad Pro 9.7 as well just a few weeks ago. Could have gone with the Air 2 perhaps, but I had the money and the 12.9 was so brilliant that I didn't want to skimp and be sorry later on. And it was a present to myself for having a more then fulltime job and getting all my points from the first year at the university and being allowed to enter a specific second year course.
When I need to pop out I take the iPad Pro 9.7 with me. Has everything, a bit cramped keyboard, a pencil and half the real-estate when compared to the 12.9, but it has all the data, LTE, I can write and type on the screen etc.
When I need to go to a client, I take both with me. Sometimes the client hasn't prepared so I can let him read on my 9.7 while I use the 12.9.
When I'm in the office I use the 12.9 and the 9.7 together. One for writing, mailing, iMessage etc, one for reading/reference.
When I'm studying, I'll use the 9.7 for reading the books/reader/assignment and the 12.9 for writing my answer/summary
Is it expensive? Hell yes, but together they where just as expensive as the top-of-the-line Macbook Pro 13"
Is it luxurious? Absolutely!
Is it over the top? I don't think so. Often at the end of the day both my iPads batteries are (almost) empty, so I use them well.
Apps I use a lot on iOs: notability, goodnotes 4, PDF expert/Documents 5, AirMail and since a few days Nebo.
Nebo is still a work in progress, but it shows soooo much promise. It works stunningly with the the main feature: handwriting recognition. but lacks several features that would make it really shine (sync, stability and better export functions are the most important to me).
One disclaimer, I still use my macbook pro with 27" screen for huge Excel-sheets and I still haven't found a really good OCR app for PDF's, so every now and then I need to do this on my macbook. Think about once a week. But it also happens that I don't use it for weeks.
That and the Nobo app would be a killer combo for business meetings.
