It's entirely subjective, but way back when, after taking an undergraduate course in Haskell, I did notice that my programming style in C became more 'Haskell-like' for a while; I used a lot of simple, recursive functions. I also noticed that this programming style seemed to yield some of the same benefits programming in Haskell had; bugs were fewer, code was easier to understand (albeit slower).
So, while learning another programming language may not make everyone a better programmer, it definitely was a learning experience for me, personally.
