I think coaster brakes are excellent for some conditions and very poor for others. On a children's bike for example they are superb. They're very simple to use and kids often have hands that are too small and weak to effectively use hand brakes. Likewise, on flat ground and at low speeds I think they are adequate for stopping and they are undeniably the lowest maintenance brake there is. If you just want a very simple bike that requires very little maintenance and always works, then the coaster brake is a no brainer.

Where I live though, we have serious hills and I wouldn't want to trust a bike with only a coaster brake to stop me on a 1/2 mile long 10% grade hill. I think one other thing to consider about coaster brakes is the person using them. A 200+ lb. adult will put way more stress on the brake than a 60 lb. kid.