The one significant piece of work required before TiddlyWiki5 can move out of alpha into beta is to upgrade the ReleaseProcess so that end users can more easily keep track of the new features.
During the beta TiddlyWiki5 will be practical for cautious everyday use but as we perfect the product there will occasionally be situations where we have to make changes that are not backwards compatible. These occasions will be clearly marked in the release notes.
Hopefully, TiddlyWiki5 will leave beta and become a full release while we're still in 2013. This is the point at which it is declared stable enough for general use. It will continue to improve and evolve after this point, although constrained to remain backwards compatible so that plugins and content created for version 5.0 will continue to work into the indefinite future.