Initially I was set on getting the 13" Air. I went to the store numerous times and felt that there was no way I could work for prolonged periods of time on the 11" screen, especially since I would be coming from a 15" MacBook Pro. However, I did a complete 180 in the last month and now plan on buying an ultimate 11".
My reasoning is simple. I spend 8-10 hours at work every day on a dual monitor Windows 7 machine. While I work at home, it's at a desk where I have a brand new 24" Dell monitor ready to be hooked up to the Air. What I really needed was something ultra-portable that I could bring with me on the occasional work trip or vacation. I also find myself in and out of meetings and in the past I have used my iPad with and without a bluetooth keyboard, which is less than ideal and at times limiting due to the nature of iOS.
Also, I often leave the office and visit a friend or stop by the parent's before heading home. By getting in the habit of bringing the incredibly small and light Air with me, I will no longer be limited to where I can do work. I have done this with the iPad in the past, but I still view it as more of a toy/couch-surfing tool than true workhorse. It's especially frustrating when I need to do work on MS Office documents or use Salesforce.com, which isn't exactly optimized for mobile Safari.
In addition this, recent rumors suggest that 256 GB will be an SSD option on the 11" Air, which will be more than enough to fit all of my applications, documents, music and photos (for now.) If 128 GB is still the limit, which I doubt, well I will have the 15" Pro at home to serve as my base.
So those are the reasons I will be getting an 11" Air.