I personally think that AST bring some very interesting things to the party. Depending on their stance they make a good compliment to both WHM and SCH, in theory. Actual implimentation depends on the skill of the player and your party mates. Personally? I think they're fun, they really do require you to be on top of things because your mistakes are more easily seen. However as you get more comfortable to them they are a perfectly fun and fine class. This is just based on my experience with them.

Though I do find it unfair to hold them directly up against SCH or WHM since the playstyle that AST offers isn't really the same as either of the other two available healing classes.