C++ is a powerful language, but with awful syntax and semantics that can discourage experimentation. Since I started using Haskell just over a year ago, my C++ code is much improved—because I can figure things out quickly in Haskell, then translate them to (somewhat write-only) C++. That I think is a large part of what people mean when they say “Haskell makes me a better programmer”. Not only is it a good language in its own right, but it also helps you prototype software that you need to implement in another language for reasons other than language power.