I use it as the scripting, and implementation, language for a console-based mail-client. (Think "mutt", but with real scripting instead of the ad-hoc support that mutt has.)

I use it because it's a simple language that is quick to code in, and trivial to embed inside a C, or C++, host application.

If I were to begin the project again I'd have no qualms about a similar approach. (Writing the core in C++ with a notion of display-modes, email-parsing, etc, and the actual control in Lua. The biggest painpoints were dealing with MIME, and similar broken emails which was largely irrelevant for the lua-side.)