I use Lua as a user-facing extension language to write modules in my music synth app [1]. Though I'm used to zero-indexing, the one-indexing never bothered me much. None of my beta testers have complained about it. I think it's a good language for beginner programmers, which most of the users are.

[1] http://audulus.com (the Lua-based modules are in version 4, now in beta)