It's my opinion that bags are primarily used as a conditioning tool and for that, it's great. But I also believe that many folk's efforts on the bag will do them little if any good, particularly if they aren't sparring, or aren't sparring realistically.
IMO, you need to approach the bag as if you were sparring an opponent.
Bags are great, though I prefer mits if I'm going to work hitting. A small, circle pad will help to develop power if that's the goal.