I initially believed this, but I have come to realize that pure functional programming is transformative for both business logic and infrastructure. Its a lot easier to get started in the business logic side; dealing with effects in the context of pure functions is a lot to figure out, I don't even know if you really can do it in not-haskell. But, via Haskell I have been able to achieve far greater levels of producitivty in these tasks than ever before.

The main downside is that its a lot to learn, and it takes a good while before you get productive. But I am absolutely more productive now in Haskell than just about anything else, save perhaps ad-hoc text munging task; the shell DSL is just so handy for random one off stuff.