So far as maintainable I'd say the internal DSLs I've made in Java as as maintainable as internal DSLs made in other languages -- that is, its very maintainable until you run into some complexity cliff.

When I program Scala or C# I tend to curse type erasure in Java generics, but when I actually program Java the generics system makes me smile. I can very efficiently turn what's in my head into a design and code.

In a language like Haskell whatever you gain from the language you tend to pay back in missing or buggy standard libraries. If I was writing a webapp in any off-brand language, for instance, I wouldn't trust that the urlencode function works correctly until I'd written some test cases.

I'll admit functional Java is a little verbose (can't write a closure in just one line) but the Hotspot compiler is very good at inlining functional code and collecting the garbage from it. I can definitely say that I get much better performance w/ functional Java than I usually see with the Interpreter pattern with two orders of magnitude less code size.