I just finished Lost Souls, so here's my review; I'm also including an overall review of Destiny.
*** Spoilers ahead ***
Sometimes it can be difficult for an author to make the third and final installment in a trilogy engaging and compelling in its own right due to the added burden of having to wrap up and resolve story elements set up in previous novels, and it's a huge testament to David Mack's skill as a writer that he's able to do so flawlessly with Lost Souls, which, I have to say, is one of the best Trek novels I've read in quite a while, and an epic ride in nearly every sense of that phrase.
Picking up essentially where Mere Mortals left off was a great way to start the book for several reasons, not the least of which being that it really allowed you to get into the heads of the Titan crew, and served as a nice bridge into the flashback portions of the story, which were essentially fragmented from the rest of the narrative.
Speaking of that narrative, I thought it was a brilliant choice on David Mack's part to make the majority of the story essentially one giant character study throughout, set against the struggle for survival faced not only by the survivors of the city-ship Mantilis, but also the universe at large.
Having gotten to know President Bacco and her staff through their participation in Paths of Disharmony, I really enjoyed that Lost Souls incorporated them so heavily into its narrative, as it lent a nice 'alternate perspective' to the story by showing us what was going on through the eyes of people who weren't in the 'thick of the action'.
I also enjoyed the portions of the novel that dealt with the battle against the Borg as seen through the eyes of Chancellor Martok and other Klingons; the Klingons happen to be one of my favorite of all the various races that populate the Star Trek universe, and Martok is one of my favorite 'secondary' characters in the franchise, so I got a kick out of seeing things from his perspective.
In terms of individual character arcs, I have to mention that I really enjoyed the stuff involving Worf and Jasminder Choudhury. In my reviews for Gods of Night and Mere Mortals (as well as my review of Paths of Disharmony), I said that she reminded me of Tasha Yar and Ro Laren, but I now have to add two other characters to that list: Jadzia Dax and Kira Nerys. The scene where she breaks down after looking at a holographic image of the tree that her father planted in front of her childhood home on Deneva, with Worf lending his support, really evoked images/memories of the relationship between Worf and Jadzia, while also reminding me very much of Kira's emotional attachment to the Bajoran faith.
I also really enjoyed the resolution of the dangling plotline involving Riker and Troi's infertility issues, and really want to see what's going to happen going forward with regards to them and Deanna's pregnancy.
Another thing I enjoyed about Lost Souls was getting an opportunity to see/experience first-hand Seven's reaction to the Caeliar's liberation of the Borg as it happened, since I'd already gotten to see/experience her reaction to its aftermath as chronicled in Kirsten Beyer's Voyager novel Full Circle.
Now, for a review of the Destiny trilogy overall:
Counting Destiny, I've only read 4 'multi-novel' Trek Lit crossovers, but, of those I've read, it is by far my favorite. I've always been a huge fan of the Borg, and therefore really enjoyed getting to see them in action again, especially since their modus operandi was so radically different from previous encounters with them. I also really enjoyed getting to experience first-hand both the genesis and evolution of their civilization. My enjoyment of the trilogy was also enhanced by the fact that I'd read it 'out of sequence', after having read three novels (Full Circle, Unworthy, and Paths of Disharmony) which are set after it, as I was able to reverse-track the evolution of certain characters and the chain of events that lead to certain outcomes as shown in those three aforementioned books.
Destiny definitely changed the landscape of the Star Trek universe, and I can't wait to eventually discover more of the Trek Lit that is set during or that deals with its aftermath (even though it's going to have to wait for a bit, as I've already committed myself to reading Before Dishonor next).