I just finished reading Greater Than the Sum. I couldn't put it down. I read the whole thing in 1 day. I loved it!
The 3 things that I enjoyed the most about the story were the cluster entity, the focus on the characters and the epilogue. For me, a big part of Star Trek is about exploring new forms of life. The cluster entity definitely did that for me. And, I liked that the story focused on the characters, their hopes, fears, motivations etc. rather than just fighting the Borg. Even Hugh's Borg provided a great contrast with the other Borg and reinforced the story's themes of family, unity, and individuality. Lastly, the epilogue really blew me away. "resistance is futile ... but welcome". wow. There is no doubt that the Borg are back, bigger and badder than ever.