Well, I know I'm in the minority but I've always found their stories intriguing. The 2 Phoenix novels, while heavy-handed with the alpha male concept, gave us a perfect villain in Omne. He knew how to manipulate Kirk and Spock and that made him very dangerous. Those novels were better than some of the others at the time like World Without End or Spock, Messiah!.
I didn't care for their take on Vulcans in The Prometheus Design but it was still an interesting look into an alien, if not Vulcan, culture.
Triangle was rather simplistic (read: Mary Sue story) with a triangle with Sola Thane. But it was a nice insight on Kirk and Spock putting each other first above their own needs.
I really loved the PHOENIX novels, and remember in Shat's first bio (written by M&C with him) he mentions that he wished they had used that for the first TREK movie instead of the TMP script. I think PROMETHEUS is good as well, but TRIANGLE was just ... well, it killed M&C for me the way the MATRIX sequels almost killed my appreciation for the first film. Thank God Diane Duane and John Ford came along, or I'd've given up on Trek fiction a decade earlier than I wound up doing.