I started using Lua as an embedded scripting language to drive a pretty complicated process. After a few weeks, i ended up switching to Python. The Lua libraries were pretty weak, the datastructures and builtin functions were wierd and inconsistent, and swig+python made for much faster scripting then exposing c interfaces to lua.
Oh, and the lack of an integer type seemed to make it really difficult to do proper bit shifting--i'd expect this to be the bread and butter of an embedded scripting language.