Kube is self-described by its GitHub repository as "a personal information and collaboration application currently in its early stages of development. It uses Sink for data access and synchronization, and leverages the KDE PIM codebase where possible."
With Kube 0.1 there is support for setting up an IMAP account, are able to read email and other basic mail tasks, but the GMail support still needs to be improved, there is no upgrade path yet, the UI is limited, and other features still need to be implemented.
Kube developers are focusing on getting the mail client solid along with working on the address book and other features. More details on Kube 0.1 via today's announcement. More background information on the Kube project from last March can be found via this earlier blog post.