You don't have to jump in 100%, you can just add formal methods to the things that require it. Cryptographic algorithms, network protocols, distributed protocols, etc, could all be completed in Isabelle or Coq and then generate Haskell from the formal spec. For less-critical parts of the system you could use property based testing and various other "semi-formal" techniques which take less time but still add to the overall quality.

I actually work in an agile environment that uses formal methods and it is possible!