Not only is this great software, but the fact that it did not ask for overreaching permissions and did not try to connect to 5 different websites on the first run made it an instant win for me.
They work for me when I walk, clean or drive. I noticed that when I do anything else while I listen, I just zone out and forget where I was and have to start over or go back.
I think many services become popular and see wide adoption, because they are free. When the free element goes away there will not be nearly as many users willing to try it or to use it and love it so much that they will recommend it.