The main times I've heard of Lua being used is as an embedded language in games. For example, Natural Selection, Cortex Command.
The LuaJIT FFI Library does look fairly nice. It appears to work in a method somewhat similar to Python Ctypes, except that it parses C declarations directly (update: it looks very similar to the new Python CFFI library). LuaJIT itself claims to be very fast and lightweight. I think there was some problem using LuaJIT in Natural Selection 2, but I can't find any information about it.