I spend most of my workday in a zero-gravity chair - I got a leather one off craigslist, but the cheap patio versions are just as comfortable (you can find a bunch on amazon). I have a monitor mounted above me and a keyboard in my lap, and it's never uncomfortable. I think for programming, being uncomfortable can definitely kill some productivity - at a normal desk, I'm intermittently thinking about and correcting my posture, or trying to shift myself to find a new comfortable position, and it interrupts my train of thought. And to feel like I'm not relaxing my ass into an early death, I'll get up and do a quick workout once every hour or two.