...but it makes you feel stupid if you don't. Better to use "Real World OCaml" if you're more interested in the ideas themselves than in their formalizations or related nomenclature.
Exactly. That's where I'm at right now; I know what Haskell is, I love the idea of it, I've enjoyed some of the fruits of it (XMonad). But it was when I tried to dig deeper into it that I felt lost, and yes, stupid. I've never been a math whiz; I am great at visualizing concepts but truly grasping the theory behind them is where I get lost. Based on my junior high school testing, I was placed in Advanced Algebra in my first year of high school. I nearly failed the class because it took me all year to grok the distributive property. I look back on that and I feel ashamed, because once I understood it, it seemed so damn simple! And so it is when I try to advance beyond my current level of programming skill; I hit brick walls and I feel like I left my sledgehammer at home. My pocketknife, even though I know every millimeter of it, won't cut through those walls.
I don't think the concepts are hard to understand, I think that - in Haskell - they're just being presented in a way that is incompatible with my way of thinking.
Having found Haskell materials as simply not suited for me I decided - quite a few years back - to learn Haskell (or the concepts behind Haskell, at least) my own way: by learning first Erlang (it sounded cool), then Scheme (mainly to be able to read many, many papers that use it), then OCaml and Scala (because the type systems and pragmatism) and finally Clean (to fill the last gaps in my knowledge). I progressed from dynamic to static typing and from eager to non-eager evaluation. It took me I think about 2 years to do all this and, of course, it wasn't that easy, but somewhat surprisingly it worked. I never wrote - and I'm not sure I ever will, but that's a completely different matter - any non-trivial Haskell code, yet I'm able to read and enjoy Haskell-related papers.
It's important to realise that there is always more than one way to learn things. You should know yourself well enough to see when the "normal" way simply isn't for you; this way you can go search for alternative ways. I guarantee that you'll find them, if you search hard enough :)
...but it makes you feel stupid if you don't. Better to use "Real World OCaml" if you're more interested in the ideas themselves than in their formalizations or related nomenclature.