I get tired standing in line or even just trying to sit with good posture in my office chair.
As a 22 year old, though, I expect I will need to make changes if I want to maintain my physical condition while spending all my time at a computer.
Five years later, I transferred to a standing desk after sitting in labs and office chairs for three years. After that three week point, once again, I noticed the uncomfortableness of it fade away. Standing desks nearly insta-improve posture if you setup your angles correctly.
The real problem with chairs: most people do not take the time to adjust the arms, height, angles, monitor angles, and keyboard/mouse position for correct posture. Usually, the adjustments are made for comfort. Temporary comfort does not equate to long term comfort. Our CIO suffers from extreme back problems in his older age.