Have you ever had a printer cartridge refilled?

I have a laser printer that I bought recently, and I’ve been printing a ton of stuff for school rather than buying books, because most of the works I’m studying are available for free download.

Anyway, the toner cartridges are about $75, so I’ve been thinking about the refill services that you see around.

Has anyone done this? Does it really work? About how much money do you save by refilling rather than replacing?