The thing that really made Haskell click for me was learning OCaml. I was getting confused about little things such as whether a particular token was a type or a value, particularly with some of the more advanced extensions enabled. Learning OCaml gave me a secondary perspective on the ML family, and now I can comfortably navigate Haskell code more or less.
I think starting with Standard ML is a good idea. It is the core of ocaml and really highlights the differences with other approaches to programming. I went from SML to ocaml to Haskell.
