Marius Eriksen (who leads a lot of the Scala work at Twitter) gave us a great quote for the back of the book that says it best.

    Programmers are digital choreographers, carefully 
    balancing correctness, modularity, concurrency, 
    and performance. Real World OCaml teaches you how 
    to perform this balancing act in simple, elegant ways.

OCaml's the simplest systems language I've ever used with a decent static type system to prevent common errors from creeping into your code (most other systems languages sacrifice modularity in favour of even more simplicity, but OCaml strikes a balance).