I've just ordered a new 9.7" iPad Pro 128GB cellular model to replace my iPad Air 64GB cellular model, which I'm using now and have for hours daily since I purchased it. It's been a great iPad and has no issues running the latest version of iOS 9.3. That said, I've been hoping for awhile that Apple would come out with an upgrade that would entice me to move on, so I could hand this model down to another family member.
This new model appears to be a really great upgrade, inheriting many parts and features from the huge 12.9" model. The downsides are the installed 2GB RAM and USB 2 Lightning port, but it gains the great iPhone 6S 12MP camera and the new photos software features that that allows (Live Photos, 4K video, etc.). The updates are huge compared to my iPad Air, and upgrading was a no brainier for me.
The Pencil and Smart Keyboard are nice new features, but neither were the discriminator for me, since I'm neither an artist nor prolific writer. It was the power of the A9X CPU, additional storage, and additional RAM that pushed me over the edge. As I said, my iPad Air has been great, but I was ready for something new and substantially more powerful to future proof me for the next three years.
The iPad Air 2 poses a more nuanced consideration compared to the new iPad Pro 9.7". The A9X blows away the A7 in my iPad Air, but is only about 15% faster in suite tests compared to the A8X in the iPad Air 2. So, if you don't imagine you'll ever use the Pencil, Smart Keyboard, or be seduced by Live Photos, the price difference may be the discriminator for you. If you do decide to go with the new iPad Pro 9.7", keep in mind that the photos it takes are going to consume 15% more storage with the higher resolution camera. That, and more movie storage, is what pushed me to the 128GB model.