Three active verisons 5.1, 5.2, 5.3. LuaJIT as 5.1 with some nice C integration features.
The most viable is 5.1 because of luajit/lua implementation choice.
Simple scripts startup time is almost the same as a C program (python is way slower to start).
Memory consumption is low. Several interpreters can be run in the same process.
Syntax is simple and yet allows to be used for DSLs, sandboxing.
RTTI is almost not available apart from discovering if the object is a function, a string, a number or a table.
Standard library is almost non-existing, except for Penlight package from LuaRocks package manager.
Documenting tools are rather lacking and buggy (tried LDoc).
Great for prototyping but as soon as a program grows beyond a couple of modules, it needs proper docs tools, higher level objects (OOP) etc. Existing solutions are quite poor.
Could not find a good REPL implementation.