I am, and I do. That is, I use Lisp at home and C# at work and I do miss Lisp when in C#. The question is a little funny because I don't consider (Common) Lisp any more "functional" than C# or Ruby or any other multi-paradigm language.
In general, C#'s functional features are becoming pretty decent, to the point where they're probably Good Enough, if a bit awkward. (Common Lisp's support for functional programming sure isn't perfect, either!) But Common Lisp is a multi-paradigm language; C# doesn't (and may never) match Lisp on syntactic abstraction, code generation, method dispatch flexibility, and so on.
If you're looking for an excuse to avoid learning Lisp because you think C# is just as powerful, sorry. :-)
