I surprisingly don't have much to say about this (except to reinforce how fantastic it is). Superman, Supergirl, the Legion of Super-Heroes, and Booster Gold all in one book? Yeah, that's a fun time right there.
My biggest thought was Supergirl's costume design for the 31st Century (in the Smallville Universe at least), mostly in how much it reminds me of the current Captain Marvel outfit Carol Danvers is wearing. It's got a tinge of the New 52 Supergirl but it really is more Captain Marvel. I wonder how intentional that was?