I like this series very much for the children I’ve used it with, but I haven’t tried it with adults. The only thing “childish” about the book is the Teddy Bear on the cover. The materials inside are pretty strait forward, so I’m sure it would work fine with an adult who doesn’t mind the teddy bears.
If you try it, be sure to let us know how it works out.
My adult studets like the books and they all are very motivated to take the evaluation afterwards. The Teady Bear never bothers them at all. I think what matters is what other songs and books the teacher assign to the students. Learning the solid techniqeu is required, playing songs they like is personal then.