I had a 2010 13inch MBA and decided to go with a 11inch 4GB 1.6Ghz 128GB flash model. I also ordered a Mac Mini with the AMD 6630 graphics card. The Mini has not arrived yet.
My original purpose for both machines, is that the Air would serve as my primary machine and I'd use the Mini as my media hub, VM testbed, and light gaming computer. I wanted to lighten my load a bit with the stepdown to the 11inch from the 13 and honestly, I love the form factor the 11inch offers.
For the entire weekend, I used my new 11inch Air connected to my 27" ACD and around the house (Sunday morning coffee on the porch/living room). I must say, I have been completely surprised with how fluid and smooth things are. With full screen mode on the Air, I found the screen size adequate. I was working a fairly large spreadsheet without much trouble. Now, it wasn't perfect, but I didn't find myself regretting the 11inch at this point remembering that I had just come from a 13.
The real surprise was the performance of the machine while connected to the 27" ACD. I could not get over how smooth things are. Websites/forums with heavy images and gif's we're extremely smooth. With my old 13 I'd always see some type of delay (albeit brief, but annoying). I could pop in and out of spaces, launch pad etc without the annoying slight jitter my old 13 gave me. I am really, really impressed.
Now, knowing the new 13 and 11 is a new class of machine all together, this really came down to size and aesthetics. For me, the 11 fits better. Maybe I'll change my mind once I am working hard on it, but for now I am happy to have this form factor. Just my two cents.