Well...I did an appraisal on a church that was proposing to add one a few years ago. It was definitely a new concept for me as well and I couldn't find any resales in my region as they were so new to the market. The only sale I found with one was also a church that had a Sprung building for expansion purposes, but there wasn't any solid allocation made between the actual church and the Sprung building. I no longer work at the place I did when I appraised it, so i don't have access to the work-file right now to get cost data from. If I remember right, I believe they were significantly cheaper than a steel- or wood-frame structure of similar utility and could be constructed in a very short time.
Through some pretty deep research, it appeared to me that they can be very functional buildings for a variety of uses. If I remember right, the frame had an estimated life similar to most steel-frame buildings and the membrane had a life of 15-20 years or so. There appeared to be a secondary market for them in the South where they have apparently been in use longer. It seemed that churches are the main users for them, although the military, schools and other similar users also find them attractive.
I struggled with the personal vs. real property issue as well, but after talking with some users of the structures and a few County Assessors, I concluded that they were personal property. Even though they have their own HVAC systems etc., they are still portable and not permanently affixed to a foundation.
The church I appraised viewed it as a temporary (5-10 yrs) solution for expansion needs until a permanent structure could be funded, even though these structures reportedly can last quite a while.
I would guess most markets would view them as inferior to conventional construction, mostly because they look like a giant circus tent from the outside. But if you see some interior pictures and learn about the actual quality of them, they no longer appear like a circus tent and aren't really that much inferior. Overall, I would say that they are inferior to typical construction, but a very good solution for someone who needs space fast.
I'm not sure I really answered any of your questions, I had a lot of the same ones and never really found solid answers either.
