I primarily used the Surface to work or "produce content." Because it is very useable in that respect. I wrote 14,000+ words in 4 days. I couldn't do that on the iPad.
I used the iPad only to "consume content." When I was done working, I always put the surface away and played with the iPad.
I used the Kindle 2 to read because I don't like back lit screens for reading books.
I used the Windows Phone for communication (when I had cell service at Lake Quinault--which was rare).
This article is a perfect example of how the tool we use shape how we use them.