Ok, lemme do a quick logical explanation of what Beta is, or is generally reffered to as far as mods are concerned.
In the sense of mods, Betas are usually relatively stable and bug free (to the extent that is possible without massive work) however all features of the game have not yet been implemented.
And this is only one release model. For example, Bushido is trying to implement everything at once (and I believe they will succeed). They will then decide if they wish to make another relase.
Anyway, in closing beta: Stable, but not yet done.
-Karrde-