I used SML as part of the Programming Languages course taught by University of Washington. It's easily one of my favorite languages and I wish there was more library support for it.
Most people don't know the object system. Most advanced features are rarely used and kept well hidden in libraries internals (first class modules, extensible types, ...)
You will need to understand functors for some libraries, though, but I would guess that it's true for SML as well. :)
Random question, how hard would it be to port OCaml libraries to [s]ml ?
The opposite is pretty trivial, though.