I started my ballroom experience with Arthur Murray out of undergrad. While I did have a bad experience with one instructor that I felt was intentionally limiting me so that they could milk more lessons out of me, the franchise owner quickly moved me to a more experienced instructor that I still actually do keep in contact with (it's been 6 years since I left). I felt I learned a lot from AM, especially on a social dancing level. I had learned most of the bronze syllabus when I was in undergrad, but still felt awkward socially when I went to go dance with a friend.
AM is really more about selling the experience than building a competitive dancer, which is what a lot of people want when they dance. It's mostly older married couples looking for something fun to do evenings or on weekends that don't want to be bothered by dance snobs which you find at some studios.
All of that said, when I started to seriously compete, I stopped going to AM and went to an independent studio for classes and invited local pros into my home to teach me on a per Diem basis. It's much cheaper and more effective that way.
Overall, my experience with AM was a positive one. I believe it's a great place for people to start... but don't fall for their sales tactics.