DS got a DOCband at 9 months. We decided to do it for a few reasons:
1) we had tried encouraging to turn his head to the other side, but he refused. he also had PT to fix the torticollis that was making him favor the right side. torticollis fixed, but he still favored the right. we thought it might even out once he was holding his head up and sitting up - NOPE!
2) his plagio was diagnosed as moderat-to-severe
3) his face was asymmetrical, which can cause problems in the future with jaw alignment, vision, etc.
4) his plagio was noticeable (even with his significant amount of hair) and we didn't want him to feel self conscious as he gets older. if he ever asked us if something could have been done, and we answered "yes, but we didn't try", we would have a hard time dealing with that.
DS's head isn't perfect and it may never be, but there is a significant change in the shape. The earlier you get it done, the better the results. He adjusted fine to the helmet and it was kind of cute on him. I have no regrets. We were able to get insurance most of the helmet. It took about 4 months and my DH's HR department getting involved, but I believe we got it covered under the "durable equipment" allowance. Also, there is a plagio group on Yahoo that provides lots of info and you can ask others about their experiences.