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
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.
[0] http://dragonboxapp.com/ ; HN discussion at http://news.ycombinator.com/item?id=4105397
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.