I'm currently working on a compile for a programming language. The compiler is written entirely in F#. The compiler (aside from the lex and parser build with lex/yacc) is basically build as a lot of transformation of a complex tree like structure.
As noted by others discriminate unions and pattern matching makes working with this kind of data structure a lot easier than dumping the code in virtual methods "all over the place"
I hadn't done any F# work before I started working on the compiler (I had however buld compilers in another OCaml variant called MoscowML) and just as Jared states it's visible from the code what parts I did first but in general I found F# easy to learn getting in to the FP mind set again after coding mainly OO for a decade will take a bit longer though.
working with trees aside I find the ability to write declarative code the main benefit of FP (F# included) having code that describes the algorithm Im trying to implement in contrast to C# describing how I've implemented the algortihm is a huge advantage.
