It was much cheaper than a decent chair + an adjustable keyboard tray. Compared to a cheap sitting setup, my legs don't feel "weak" and wobbly after longer sessions, I don't slouch, and the keyboard is in proper height. There's also much more space for crap; for example, I can put a cup of tea on the bottom desk without any risk of spilling it on my keyboard.
I'd appreciate a stool every now and then though… like whenever I have to code all day long.
I like that I can walk around and think without any overhead (= standing up).