After a few weeks of using it, the answers are simple: it works really well, and other than my feet being sore at the end of the day, I'm really don't mind standing for a large part of the day.
All of the things I had read about standing while working were true: my posture was better, my energy was better, overall I just felt better. But there were some more unexpected side benefits.
For the past several years I have used two monitors. With my laptops I have used the original Griffin iCurve and a second monitor, and even with my iMac I have attached a second monitor as shown above. Just about everyone who sees it for the first time asks why I have two monitors. It usually takes less than a minute for me to show them the benefits. This was actually my primary concern about getting a standing desk: would it be a) strong enough and b) large enough to support both monitors?
As you can see, it's a tight fit to put both the iMac and a second monitor on this particular stand, but I have no worries about it holding up as it seems very solid. What I did not expect was that I use both monitors more equally now that I used to. Maybe because one of the monitors is smaller than the other (see note below), I always tended to use the smaller one as a side-glance monitor: it's where I'd put Echofon, Adium, or Mailplane but my "real work" was done on the iMac monitor. Since I began working while standing, I find myself switching back and forth to use them both equally. The only explanation I have is that it is easier to make the minor adjustment to be looking straight on at either monitor than it is when sitting at a desk with your legs confined to a certain designated area.
I also find that I move around more during the day. Since my standing desk doesn't have a lot of extra room on it, I tend to only put in front of me whatever I am working on right now. I have a whiteboard that I never used very much before that is now just a step away, making it easier to use. My old desk -- your standard 1950s wooden desk with drawers -- was never particularly well suited to a computer; it didn't even have a keyboard drawer. On the other hand, that old desk works extremely well for sitting and reading or writing by hand (yes, people really still do that). I find myself looking at my tasks and thinking "OK, well here's something I can do sitting" for when I need to take a break.
When I've having trouble focusing, I've used the (10+2)*5 to try to force/trick myself into a more productive mode. I now use the 2 minute "breaks" for sitting down, checking email, Twitter, or Tumblr on my iPhone and turning off email, Twitter, and Tumblr on my iMac. This has been my habit for the past few weeks: I get into the office a little before 9 am, and give myself until 10 to check in with the secretary, check email, and catch up with whatever I feel like I "need" to read (Google Reader, Twitter, Tumblr, email).
Then I set the SelfControl to block Twitter, Tumblr, and email from my iMac until the end of the day. That leaves only one option: my iPhone (or, someday, my iPad). During those 2 minute breaks I can check any of those sites on my iPhone while sitting down. When the timer goes off I'm ready to start again and stand back up. Having two distinct modes of working (standing and sitting) has been very effective for me in terms of staying on task. If I'm standing up I want to make it worth the effort. Slumping in a chair is much easier. It's also amazing to me how much less time any of those things take when you sit down and go backwards through them, rather than keeping up with them as the day goes on.
The BusinessWeek article talks about several different health factors related to sitting instead of standing, but to me the most noticeable ones are mental focus (my mind seems much sharper) and posture (I don't have the back aches that I was used to at the end of a long day hunched over the computer).
1) If you are thinking about a dual-monitor setup, I highly recommend two monitors of the same physical size and resolution. That's my only real complaint about my current setup.
2) I have ordered a GelPro mat ($100USD) to reduce foot fatigue.
3) I asked a nurse friend of mine to answer some ergonomics questions which she answered on her website.