I use unmedicated. I buy all my feed from an organic-only place that doesn't have medicated, so it wasn't really an option (I mean, without going somewhere else, obviously). In general, I don't believe in treating for stuff before it happens, or in over-treating-- I don't put flea/tick poison on my dogs, for example, if I haven't seen any evidence of fleas. I'd rather give my chicks unmedicated feed and just keep a very close eye on them, and treat if I see any signs of illness.
That said, I have a very small number of chicks (3), and got them from a small-time breeder. If I had a lot of birds, or if I was getting them through a hatchery or big box store or something, I might go medicated. It really depends on situation, just like everything does, really.