If it wasn't for Peter David, this book would have scored a "70%".
While I thought it was interesting enough to keep reading, Vendetta didn't leave any lasting impressions on me.
An important character in Vendetta was Guinan, along with her half-sister, Delcara. Suffice to say, Delcara was not very likeable - borderline annoying. She was extremely one-tracked-minded on the destruction of the Borg - due to her past dealings with them. Hence the title, "Vendetta". At the very least, she was developed very well.
Unfortunately, as far as the actual plot goes, the television series Star Trek: Voyager takes two different aspects of this novel, and out-dates them rather quickly. In Vendetta we see the Enterprise-D capture a single Borg woman and then they try to "de-assimilate" her from the collective. She was not able to be restored. In Voyager, we saw the same thing happened, but with different consequences: because they were successful, we end up with series regular, Seven of Nine. Secondly, Voyager introduced Species 8247. A species that had no trouble dealing with the Borg whatsoever. This novel also introduces a species that can likewise handle the Borg rather easily. This cheapens the effect on the story. I suppose when it was written - in the early 90's - it was very original. However, it should be noted that I read this book prior to ever seeing Voyager and that didn't help change my opinion Vendetta. So it may depend on the reader and the preference of the subject. In each case, I was disappointed to see a species easily overpower the Borg. That should never happen.
This novel lacked the typical Peter David humor that would have made it that much more of an enjoyable read. Of course, there was one sub-plot that had his fingerprints all over it - and most would find it very humorous: The Borg assimilated a Ferengi and tried to use him as their speaker, similar to Locutus. Although, it was not intentional humor, there is just something funny about mixing the Ferengi with the Borg.
The ending was fairly interesting and it kept my full attention, but the rest of the book was a bit slow for my liking and the sub-plots left alot to be desired. (i.e. Geordi trying to humanize the Borg drone...) Although it was interesting to see Commander Elizabeth Shelby working as first Officer to Captain Korsmo on the USS Chekov. It provides some basic background to the Star Trek: New Frontier novels. We see Peter David took a liking to Shelby as a character early on.
So it is safe to say, Vendetta was a decent novel - most likely great for it's time (being one of the first "ambitious" Trek works) - but it is by no means a top priority as much better novels have come out since.