Yesterday I installed workrave on my box to test out versus my long computer binges. While I haven't ever really had any problems due to this at this point in time (and shouldn't at my age, jeez), it does get somewhat uncomfortable after a few hours of not getting off my chair. So far, having a reminder to get up and stretch/walk around for a bit works well.
I like its customizability, too - I turned off the panel countdown timers as they were exceedingly distracting, and tweaked the time between breaks up somewhat to a 30-second micro-break every 15 minutes, and a 5 minute rest break every hour. It seems about right so far, though I'll keep an eye on it for a while. Little bit memory-hungry for a little applet, but I suppose not too much worse than other applets.