After trying a non-adjustable standing desk that I built for a period of two weeks, I came to a similar conclusion. It initially felt pretty good, but after a few hours had passed I ended up taking a lot of breaks that I normally wouldn't have, simply because my legs couldn't keep up.

Also, my feet were in some serious pain for the first few days, even with a thick floor mat, and continued to lightly ache afterwards.

Anyone thinking about doing something similar may want to read this brief article about some of the potential risks in standing up all the time: http://healthland.time.com/2011/04/13/the-dangers-of-sitting...