Chiming in a bit late, but I just got back on the forums (life took a twist --what's new!?). Sonotubes ARE a great option IF you do them correctly for your building, weather, and soil.

Here's a 14 x 24 that I'd love to build: http://www.countryplans.com/nash.html The website talks about how to insulate piers, etc. Best of luck!