This is a weird one to review because I don't know the Legion, and without knowing the Legion, a lot of this is just a big, messy fight scene with some dimension traveling thrown in.
Superboy Prime is back, and after getting a lesson on the future from a Superman museum, he decides on a new goal - gathering villains and attacking the Legion with an eye toward destroying them.
I do at least have some knowledge of the events of Final Crisis. Without that, I think I would have been totally lost. There are some comics - a great many in fact - that you can start with and where the standing cannon doesn't hurt your understanding of the events in the book. This is not one of those books. Even having a decent background in comics in general, most of this simply didn't grab me or mean anything to me. It's essentially one long fight. There's a ton of characters, so none of them get much development beyond "they must be friends/in a relationship because when one dies the other is sad." It's a book meant for people reading all the other Final Crisis tomes or longstanding Legion fans, I think.
That being said there was one part of this I enjoyed massively - specifically the return of two characters I used to love before the new 52. (as they are now, no thank you. But back then, they were amazing!) But it just wasn't enough to carry this book for me.