People are often skeptical of the usefulness of 150, due to the fact that the SML language is not a language that is ever really used in any industrial context.
While there are a great deal more rebuttals I could give to that line of thinking, one fact remains, which is that students of 15-150 effectively know another language automatically. That is the OCaml language, which resembles SML greatly, and is used for various real-world applications.
This lecture goes into the differences between SML and OCaml, and the reasons why OCaml can be a nicer language to work with. It aims to bring a student up to speed with being able to program in OCaml at a similar level.