Why you should use a coaster brake: Because you're too old, fat, or young to go fast anyway, you live where it's basically flat, and that's what came on your bike.

I'm not knocking coaster brakes, I've been riding the cruiser there in my avatar for a couple of years now, and it's a coaster brake bike. But if you have a frame made for any other kind of brake, I can't think why you'd want to downgrade to one. They work within limits, but if you don't fit the qualifications above, just use the rim brakes your bike was intended for. If you have to ask if a coaster brake is going to be good enough, that's a pretty good indication to avoid it.

And if you're interested in coaster brake bikes, check out www.ratrodbikes.com, lots of cool bikes there.