I'm going to preface my comments by saying I have not received ASTYM.
Receiving ASTYM is probably better than receiving no soft tissue work. It looks eerily similar to Graston Technique, but with less pressure. This may or not be controversial, but I believe PTs aren't very good at soft tissue work. They just don't receive enough training in it. Massage therapists are trained to feel with their fingers, hands, forearms, elbow, etc. PTs who use tools in their soft tissue work do not have a good sense of what's going on underneath their tools. Many massage therapists also have no idea how much pressure to apply either, so it's hit or miss depending on the training and skill of the massage therapist.
If I were you, I'd prefer to go to a good manual therapist trained in orthopedic or myofascial massage. But, ASTYM is probably fine; it's just not as good as what you can get. If you do go with ASTYM, don't be surprised if it hurts.