Just finished and after some internal debate rated it outstanding.
At first I was just going to give it an above average. But after some thinking there was one important thing that pushed it up to the top.
A unique point of view.
Here's the thing, as Trek fans we are on the "inside" we look at the events of things like Little Green Men, or Assignment: Earth knowing for the most part who every one is and what is going on. While there might be characters and moments that at first we are unsure of for the most part we know who the good guys are and who the bad guys are, and we know how the future within this reality turns out.
But take away that knowledge. Put yourself in the shoes of someone who's only certainty is that there are alien beings (after all he's met them). In the fullness of time he learns that some aliens want to help, but some don't. Some want to destroy us. Imagine what a frightening place the world would be with that knowledge. Now even worse imagine that most everyone else has no such certainty on the matter and even worse you are restrained from ever sharing what you know.
Suddenly the history that we know takes on a new darker tint. Suddenly the fear and suspicion with which Starfleet officers from the future have been greeted with by people in the past doesn't seem quite so silly or backwards. Sure *WE* know that they are the good guys. But for people living in the moment they don't.
Another delight of this book for me was that it was my favorite type of Trek novel, namely a connect the dots story where multiple episodes are connected together in a coherent narrative framework. CTD novels are not easy to do well. Often they come off as interesting but not really standing on their own merits. In some ways I will confess that I almost wish this novel had stayed rooted in the past and left off the "present" day bits, but it's a small wistful comment and not a real complaint.
Over all it was very well done, the characters were well drawn, and I was so caught up that even I was caught a bit by surprise by Voyager's appearance in '96.