Like many here, I too just finished reading Ex Machina and found it to be quite excellent. The characterization of the main cast was sharply authentic and the development of the new cast of background players was equally deserving of praise. Many Trek books have shown us what happens next after the Enterprise departs at the end of an episode (and this one does, too) but in Ex Machina, we find out what happened BEFORE the start of "For the World is Hollow..." as well. Boy, do we ever!
This aspect -- the delving into the past of Yonada and her people -- is what I believe puts Ex Machina into an elite class of Trek fiction. A lot of time, imagination and research went into this book and it shows on every page. I used to think that Christopher Bennett was best suited as the primary writer of the Titan series (since those were the books I most enjoyed reading from him) but now that I've read Ex Machina I may need to reassess that idea. Perhaps his talents would best be served by authoring more post-TMP Trek stories. If we're able to read more of either -- post-TMP or Titan -- in the near future from Christopher, I will be one happy reader.
In summation, great book. If you haven't read it, do yourself a favor and join the ever-growing Ex Machina renaissance by reading the hell out of this novel. You will be so glad that you did.