I purchased an 11" MBA last year and my wife got a 13" MBA (yes there's a comment there

). I was a little apprehensive about getting the smaller screen as my previous laptops had all been 15" and I was used to that size.
While my primary work is done on two 22" screens and I could not really imagine doing my day job on the 11" screen. But everyday duty falls to the 11" MBA and it does it wonderfully.
While the screen size and resolution are not great for my day job, the use of full screen applications make it perfect for multi-tasking. I can work on email, web browsing, blogging, instant messaging and doing basic coding within a UNIX env without a problem. Even using photo editing programs does not seem to be a problem with the screen size, mind you for proper colour calibration I'd rather the two 22" screens
The weight is perfect, and the overall size is great for use in cafes, waiting rooms, terminals, on planes, in lectures/training sessions without sacrificing too much room, not to mention lazing in an arm-chair or sofa. It has been knocked off the arm of the sofa by my Son (it was an accident) and suffered no damage.
The battery life does leave a little to be desired, but it is better than the previously mentioned 15" laptops I used to have and I still get 4 hours with ease.
In the almost year I've owned it I have never once wished I had the 13" version, and have never reached for my wife's 13" MBA to do anything even though it has the i7 versus my i5 as well as the additional screen real estate.
Overall if you go into the store look at both and decide you can work with the screen size and resolution in what you need, then go for it, you will not regret it.