Mostly play. I sing in a chorus, and I keep my sheet music on PDF, the larger screen would be nice. Web browsing and email, Netflix, and some light MS Office work.
Digital photoframe at work.
As it is, as light-weight as all of that sounds, most of it crashes on my my iPad 2 now.
I'd really like to see how much iMovie editing can be done on the fly and how it integrates with iMovie on an iMac for polishing. If also like to push the boundaries of using it as a replacement for my Windows Laptop.
Definitely going to be using the multitasking spilt screen and the new pencil to draw going to be a huge help being an artist to have an all in one device for everything.
I think the iPad Pro will be great for multitasking with that huge screen - I can work through emails and add/complete tasks in OmniFocus on one big screen.
I'm going use it at work to read and annotate papers, but also with the Duet app to give my work iMac some more screen real estate, since my boss is too cheap to buy me an external monitor. Even that little bit of extra screen will help. And I'll use it at home to sketch.
Like other posters, I am looking forward to trying it at work; I'd like to be able to take notes at meetings on that big screen and do some image editing to show others during collaborative get-togethers.