I also reply to emails and schedule calendar events from it. As soon as I need to edit documents, manage servers, or write code - I move to a laptop or desktop, it's just more comfortable.
I've been using one of my tablets to read through a book I'm using to tutor a friend's high school aged kid.
My wife uses one to read programming books and reference materials.
My preschool-aged son and nephew watch educational videos (type "number", "polygon", "alphabet", etc. into the youtube app; there's some great stuff out there.) They also play number, letter, color, shape, reading, and math games (the five year old plays Dragon Box! [0])
My sister uses hers to schedule substitute teaching and tutoring jobs, and on occasion she works as a manuscript editor (the keyboard dock is important for this.)
So there's a lot of reading, learning, and even production of content going on on tablets in this household.
I don't think reading/learning are productive use cases. Productivity, IMHO, means you create/produce something that is tangentially different (regardless of how small it is) than what existed before an activity. In the minds of most people, creating new knowledge or acquiring new information does not conform to the above definition.
You could argue though that iPads and other tablets _help_ you stay productive on a broader basis. At least that's how I justify my use of my iPad. They simplify certain aspects of my workflow (like easier/intuitive annotation of PDFs). But I know that simplification is not at a level where I'd like it to be.
So - reading/learning. That can be productive.
I also reply to emails and schedule calendar events from it. As soon as I need to edit documents, manage servers, or write code - I move to a laptop or desktop, it's just more comfortable.
P.S. wrote this on my iPad