I have an iPad 2 and a late 2010 MBA 13" which I only use in clamshell mode with an external 25" Dell monitor 99.99% of the time. I recently got an iPP 12.9" with Smart Keyboard and Pencil to see if the iPP could be a replacement for both devices for me. The first two weeks after I got it, I never touched my MBA once. Kinda missed the 25" monitor a bit but the luxury of doing all my computing from my the comfort of my bed was a nice trade-off.
Then I needed to download something on bittorrent. Had no choice but to wake up the MBA to do it. Then I was getting back into digital electronics and needed some schematic and pcb layout software. While there were a few apps that you can draw pretty decent schematics on and even run basic circuit simulations, the component libraries aren't extensive and you can't easily add custom components. And there were no pcb layout apps available at all. So again, I was forced to use my MBA. Lastly, I'm a freelance developer and while I could conceive of a way to do all my web dev work just with an iPad, I still need a Mac when I want to bust out something in XCode.
So ultimately, even though I love this device and it's sorta capable of being a tool for some production work, it's not 100% there for all cases yet and I personally can't justify the cost of owning both an iPP and a Mac. It has to be one or the other so for me, I'm going to sell the iPP and get a new Mac. Especially the way how iPP got completely ignored at WWDC, makes the decision to sell the 12.9" much easier to live with. There was nothing announced in iOS 10 that would make my workflow easier on the iPP which is very sad.
One thing I'll say is that while split-screen and having a physical keyboard with an iPad makes it much more usable as a work device, sometimes you just can't beat being able to have a dozen windows open that you can resize and position anywhere you need them. And hovering your hand to make multiple taps to accomplish a simple task can get very tiring and uncomfortable. Especially for things where you'd normally use a mouse to draw paths or make a few quick clicks, things like that. Just having to reach your hand up from the keyboard to tap something all the time can get old fast. With a mouse or a touchpad at least your hands stay on the same horizontal plane so you don't fatigue. I guess YMMV but for me I found it a bit of an annoyance. If you like the way you do things on a laptop, get a laptop and if you don't need specifically to use Apple Pencil save money and go with a refurb Air 2.