I wouldn't say experience with imperative languages is useless for learning fp. Knowing the basic types (int, char, bool,...), the concepts of operators and precedence (ok maybe that's just math), understanding the difference between a function call and a function definition (something that absolute beginners often have problems with), and many other things that are so natural to you now, are still usefull in haskell.

I don't know (yet), if learning haskell will be usefull for programming in imperative languages for me. As others have sayd above, I think that learning to think in new ways is always usefull in the long run. And maybe it will prove usefull to apply some patterns found in fp in an imperative language. Like recursion, folds, etc. I'll see.