I think that Lua in Rockbox as lot of potential, specially because it doesn't require you to build a custom build to run a LUA plugin, it isn't affected by minor changes of the Rockbox version ("normal", C plugins report "Incompatible version"), it's much easier to share and cross-target (if the script isn't affected by the amount of memory or screen size of the device, you can just copy it to a different Rockbox player and it will run that script too). Also, Lua is, in some people's opinion, easier to learn and code than C.
In the sequence of this, I tried to develop a few simple plugins, to get one conclusion: there's no real documentation on how to use the Rockbox API to show usual things as menus. I added another code sample on the Lua viewer wiki page, which shows how to use the Menu API, and I plan on coding more sample Lua plugins as both a way to create more documentation on the subject and to learn Lua at the same time.
Lets hope that with these contributions, more Lua plugins are coded for Rockbox...
By the way, Lua can not only be used for coding plugins but also less elaborated scripts. For example, I made two very simple scripts that turn on and off the CPU boost, just by launching the Lua file on the browser.