Everything is in the title, all the preview features are now stable and available and no-longer in beta.

RStudio Blog: New Version of RStudio (v0.99) Available Now

Lots of new features:

• code diagnostics
• snippets
• support for diagrammeR & GraphViz
• vim mode
• improved code completion (including dplyr piping support!, works so nicely in the beta)
• improved data preview

And probably more I forgot right now, go read the blog and try the release!