You can get a lot of mileage out of functional programming as a style without using a functional programming language, and you can do it in (almost) any other language, the big one: avoid side effects to functions as much as possible. That alone can make all the difference between a maintainable, refactorable and testable codebase compared to one that has side effects all over the place. If that's all you get out of it that's profit.
That's always been good programming practice to me, not functional per se. I learned the term 'spaghetti code' early on and took it to mean not just gotos, but lots of flags, especially globals.
