This one picks up where Full Circle left off. This isn't very spoilery: Voyager returns to the Delta Quadrant (this time, with the "Full Circle" fleet) for a new mission of exploration, and a lot of our characters are back, albeit most in different roles.
I liked this one better than Full Circle mostly because the Warriors of Gre'thor story (which I was dumped into, in medias res, in the previous novel) did not play a role besides a brief mention. The story is indeed far more coherent in Unworthy. Everything revolves around the discovery of a new species with a mysterious connection to the Borg, and all the ongoing character arcs intersect nicely with this plot and advance satisfyingly as a result of the plot. What's more, the plot has a beginning, middle, and end all in one novel! (Yes, this is the kind of thing worthy of note in our present age of trilogies, quadrilogies, decalogies, and so on). In short, Unworthy occupies a good middle-ground between tying in with the rest of the post-TV Voyager book series and being a stand-alone story that can be enjoyed by someone unfamiliar or only vaguely familiar with the happenings of the Voyager crew after the TV series ended.
As far as character arcs go, they're all pretty good here (if you read my review of Full Circle, you'll see that this is not the case with that novel). The best, in my opinion, is Seven's arc -- she's struggling with an identity crisis that goes beyond that found in the TV series, and we really see some character development here, enough that I'm curious to see how she develops in future Voyager novels. Chakotay, Tom, B'Elanna, and Harry get a lot of page time in this one, and Reg Barclay even gets a few exciting moments. A guest character from one episode of Voyager shows up too, quite unexpectedly, and I'm not entirely convinced that that part of the plot was necessary... but it was at least interesting.
Overall, it's a pretty good Voyager book, and I recommend it to Voyager fans especially. You will probably enjoy it as a stand-alone, but it's still better to at least read Star Trek: Destiny and Full Circle first.