How useful are functional programming languages?

I hear they are good, make it easier to maintain code-bases. Most often I reach for python to get the job done. Does anyone have experiences with functional languages for larger projects?

In particular I am interested to learn more on how to handle databases, and writing to them and what patterns they come up with. Is a database handle you can write to not ... basically mutable state, the arch-nemesis of functional languages?

Are functional languages only useful with an imperative shell?