I have had a standing desk for about seven years. For what it's worth, I only work at a desk about 10-15 hours per week.
I have found that I feel best when cycling between standing, sitting and lying down. In fact, there are certain kinds of work that I find myself completing more efficiently and enjoyably in different physical configurations. I like to stand at my desk for phone calls and support interactions but I like to lie down on my couch for research/reading.
I have indeed found that standing for 4-6 hours doesn't feel all that great, but in different ways that sitting for 4-6 hours doesn't feel great. So I have a standing desk, a chair, and a couch and cycle between them depending on what kind of work I am doing.
You do you, I'm glad it works, but sit/standing can be done healthily too.
I don't think sit-stand desk is such a good idea. Sitting and standing for long periods of time is not good for your health.
Here is the way I work, it's like a game...
+ Work in 30 minutes intervals where you only sit.
+ Have a timer (e.g. Google Calendar) that let's you know your time is up.
+ To get another 30 minutes of work, you have to workout for 2-3 minutes.
+ Have a set of 10-15 pounds dumbbells next to your desk and just workout for 3 minutes. My favorite workout is "goblet squat". You can learn the proper forms from YouTube.
Benefits:
+ You have more focus!
+ You won't waste time solving problems that does not matter because you have worked hard for that 30 minutes ;)
+ You'll become stronger! Your body starts to make muscles.
+ You will never get tired from sitting!
+ You'll be ... OPS... my time is up I have to workout...