As far as sidepulls go, they are great for training and ground driving, and all around riding. I do use a sidepull attachment... which I got off of ebay
(her store name is crazy ropes by debbie I think) I love it. My horese responds to it real well. I ride bitless, but I also train bitless. If you have a well broke horse, the transition from bit to bitless or sidepull should be simple. However, if you have a horse that may not transition smoothly, I suggest ground driving in a regular side pull for a few days to a week before using one of these attachments. For the person who said nobody rides in a bosel... you are wrong. Many people use them. We are totally bitless. For the most part, I use the sidepull attachments (after I initially break the horse), however, I do have 2 horses that I use the indian bosels on. They are great.
What I love about the attachments is, I can use any headstall I want. They are beautiful, and then being able to use your own headstall... its just great. Same with the indian bosels. So no, you do not need to buy a special headstall, you can use your favorite one you have now!