While I've owned 13" MBs and MBAs in the past and own an 11" MBA now, I was torn between an 11 or 13" 2012 MBA.
When I went from a 13" MB to a 13" MBA even though the footprint was the same, the Air was about half the thickness and weight and that made a huge difference in mobility. The 13" Air was a great improvement and by far the best notebook I had ever owned to that point.
When the 11" came out, I was a bit worried about the screen size, but it's form factor was so compelling I had to try it. It's been a great travel machine. While its screen size and battery life hinder its all day useability, it's a great machine to travel with, use in bursts through the day, or use with an external monitor when in a fixed location. My eyes aren't great but the screen is so crisp and clear its never bothered me, even though I have to be pretty close to it.
So I went to the Apple store on the weekend to make up my mind. If I needed something to use all day long, without an external monitor the 13" would definitely be my choice. But as I have external monitors available where I would work for long periods and the smaller screen is just fine for shorter use, it's still in the running. Battery life is not an issue with my current 11", it's form factor is fantastic for travel, and it's small enough that I don't have to take it out of my luggage when flying, so an 11" won out in the end.
My local Apple store did not stock the configuration I was looking for (11"/2ghz/8gb/256gb) so I ordered online today.