I read this when I was twelve, and being one of those poor young Trekkies whom the Borg captured the imagination of, I read this book repeatedly.
Presently, I think it's an interesting story but an excellent example of why, as an adult, I can't stand Peter David's writing. Vendetta is very much written in the style of a Star Trek comic book story (which happens to be the mainstay of what David writes for a living). By that I mean mostly that David takes great liberties in his depiction of the Star Trek: The Next Generation characters (both the crew and the Borg).
The content of Vendetta is also extremely self-indulgent on David's part, in that it:
-plays favorites with certain regular characters while blatantly ignoring others (Riker might as well have been left out, and here and in other Star Trek novels he wrote, David obviously doesn't think much of Data)
-makes gratuitous pop-culture references, such as calling the Tholian web ships "Tholian Webslingers" as a reference to Spider-Man because he has edited Spider-Man comics)
names a character after one of David's daughters even though the character was depicted as male on television (one of PD's later books apparently offered the excuse that the new character was the television character's daughter)
-includes a sexual anecdote of questionable relevance
Peter David's style is consistently and self-consciously postmodern. Postmodernism doesn't agree with me, so I avoid David's work like the plague.