We are following the Linux model of releases, where an even second digit of a release number indicates a "stable" release and an odd second digit indicates a "development" release (e.g. 3.0.x is a stable release, and 3.1.x is a developmental release). If you are using a developmental release, you might get to play with extra funky features, but it is entirely possible that these features come/go/transmogrify from one release to the next. If you require stability (e.g. if you are using Weka for teaching), use a stable release.
Machine Learning Group at the University of Waikato