Almost everything is rapidly evolving. There has been some major refactoring between 0.1 and 0.2 and again between 0.2.x and 0.3. This will continue apace until we're much closer to a 1.0 release. If you want to get a sense of how stable something is, one heuristic is to have a look at how many unit tests there are for it, and whether those tests pass.