DejaVu Sans Mono is a must have (or a must try at least). It has 10x more Unicode glyphs than any other monospaced font out there, and is easy on the eyes (close call with Monaco on readability I'd say). Menlo is just an Apple-botched version of DejaVu (saying this as an Apple fan btw), without the extra glyphs.

With DejaVu, you can easily get awesome things like: ▷ λ ⇒ ≤ ⊥ ⧺ ∈ ∆ ⤜ ∅ ∑ ≡ ∘ √ (all in true monospace that plays nicely with Vim!). I have to admit this may not be for everyone :), but fun to hack Haskell with.