Dark Beasts aren't bad; they are just decent experience, nothing more, nothing less. As for the drop rate of dark bows, it is low, very low. I'm 9 Dark Beast tasks in a row right now without a dark bow. Their drops are alright, including several alchable rune items that will pay for your prayer pots. I wouldn't suggest camping these though, as without a black mask it will be painful to kill them. I've found they have a very high defense to the point where you can kill them roughly 20% faster using a gs over a whip. The charms are alright; they are the best blue charm dropper in the game, but its not as good as hunting waterfiends. And yes, they are aggressive forever making them easy to afk; just don't let your prayer run out or you will become quick food in your proselyte. Is it worth it? No. Is it worth it if you enjoy training the skill, and would eventually want the cape? Yes.