MySQL has introduced Previews as a new way of delivering features for early testing by our community.
Previews are for testing of features, not of versions. The two previews we offer today are the Falcon Feature Preview based on 6.0, and the GIS Feature Preview based on 5.1. However, the version number as such is not intended by us to set any expectations on when the feature will be in a production version. For instance, Falcon is going into the 6.0 code base, but the GIS geographical data won't be in before 6.1 (although the preview is based on 5.1).
Previews are by definition a bit "rough around the edges". They have no inherent level of maturity -- they aren't pre-alpha, or alpha, or beta, or RC, or GA. They are a new way to do "release early & release often" for those who want to get things early, without MySQL tying itself into a promise on the specific delivery release.
Falcon 6.0.4 is available immediately in the preview release and soon will be part of a formal beta. If you want it immediately, go get the preview version!
Our purpose of having previews is to make new features available for those who want early access. That way, we hope for more community testing. For our users, Previews are meant for testing only, absolutely not something to use in production. It's a new way for us to release early & often for experimentation, learning, and testing.
Later on, I hope we will do other feature previews for storage engines or specific features.