Big things indeed.
I really want to go spoiler-happy here, but I'm going to restrain myself.
David Mack has really constructed a brilliant trilogy here. Major changes for the Star Trek universe as we know it, and all the doom you'd expect from a major all-out offensive by the Borg, while still retaining that special Star Trek optimism and wonder for the universe.
This book brings what has turned into a six-book epic about the Borg to its fruition. Some people have been complaining that the books have used the Borg too much over the last two years, and I was starting to agree with them, but the Destiny trilogy puts it all in perspective. The Destiny trilogy, along with the Next Gen books Resistance, Before Dishonor, and Greater than the Sum, are Star Trek's equivalent of Star Wars' New Jedi Order series. Major deaths, new characters, and major ramifications not just for the heroes but for the entire Star Trek universe.
This book is not only why I read Trek books, it's why I can't stop reading all the new Trek books. The best Star Trek books of all time are being written right now, and this trilogy is at the top.