Learing Ocaml when you know Haskell?

I know there are a few useful programs written in Ocaml and that John Carmack—praise His name—uses it.

Are those reasons enough to learn Ocaml?

>>1
I hear talk of "Real World Haskell" and all I can think is U MENA OCAML?

Seriously though, are you unhappy with Haskell for some reason? If so, OCaml may be able to fill that void without compromising what you like about Haskell too much. In general the answer to the question "Should I learn this member of the ML family?" is always "yes."