I posted this in another thread - I've had both the 9.7 and 12.9, and between the two, I definitely prefer the 12.9, but neither are right for me:
Here's my story/mini review:
I had an iPad Pro 12.9", 128 GB w/ LTE. Paid $550 on Craiglist, and bought the Pencil. I was happy as a clam... I passed the Patent Bar thanks (in small part) to the notes I took on the 12.9" screen, and loved the fluidity in Notability and Notes. I sketched more than usual (which is still not much) and tried to read a book or two (though I much prefer my Kindle Paperwhite).
So I was happy with my Pro! Until I realized what I used it for all but 30 min/day.... Reddit. iMessage. Occasional email (though I'd jump to my MBP for a long response). Random sketches while I watched Seinfeld. A $1200 (at retail) iPad Mini 1. So I sold it on Craigslist - $750 for the package with an iPad Mini 2 (16 GB, LTE) included (!). Needless to say, I was happy to skate away with some extra money in my pocket, and an iPad that does exactly what I wanted it to do.
I've had both iPad Pros now, and they are truly amazing devices to use. It feels like the future. But for me (and I do not speak for anyone else), they don't replace my MacBook simply because I enjoy using my Macbook more. Could they? Sure, I think so. I don't do that much intensive stuff, and the iPad Pro lineup is very capable. There are extremely few tasks (the rare molecular modeling experiment) where I use the quad-core capabilities of my laptop. But for 99.99% of tasks, the Pros have the power. They just don't have the experience I, personally, am looking for.
(By the way - the Apple SIM does indeed work in the Mini 2. The Apple website does not denote you're able to do this, but I can confirm it works!)