I haven't worked as a runner but have had some work for me. It can be a great way in although it depends who you get to work with, it basically makes it possible for you to get to know more established people, like if you want to be a DP, you can try and befriend to DP (or one of their assistants), make an impression and they may offer you a position on the next film they do as some kind of second assistant and so on.

Generally runners make coffee and go out to fetch food and random stuff like tape that people forgot or ran out of, on lower budget sets you'll probably get roped into all kinds of other stuff like moving stuff around, holding up bits of falling over scenery, pushing the camera dolly, whatever crops up really, sometimes it's incredibly boring sometime's it's incredibly fun, but to make it worthwhile you have to network, do a good job and be great to work with. There's a funny British site called roguerunner.com I think, on it there's a "diary" kept by the site owner when he worked as a runner, check it out.

Steve Piper
Coffee Films
www.coffeefilms.com
www.myspace.com/coffeefilms