I work a desk job, only about 20 hours per week or so. I don't have any pain so to speak, but I just feel tired often after work from sitting all day. I get up and walk around a lot, or walk somewhere for lunch and it seems to alleviate any stiffness I get from sitting. I hunch over when I sit too, which obviously is not a good thing.
I do find that sitting a long time, often depsite the comfort level of the chair, makes my lower back hurt a little bit. It's more stiffness related just from not moving.