I'm using GMail contacts as my primary contacts database. It synchronises with Google+ and Android very nicely. For now, I just export my contacts there and import them in Roundcube to keep them consistent. But it would of course be easier if a GMail contacts import/synchronisation would be integrated in Roundcube already. Could this be a plugin or something, using Google's app authentication mechanism like other websites do it?
(Oh, and don't forget the pictures, since Google can't export them through their UI...)