I've decided that I really, really need to get a speedlite flash for my XTi. Since I've generally had decent luck on eBay, I wondered if that was a wise way to get a used flash. My concern was with the expected lifespan of a flash. How long is something like a 430 or 580 supposed to last? Would this be like buying a used light bulb, or do they tend to last many, many years, making a used flash a potentially good deal?