I was a big fan of Serpents. Second to KRAD, it was one of the standouts in the Lost Era series. (Though I did like both the Sulu, Garrett, and Sisko/Tuvok novels; I picked up Christopher's novel but haven't read it yet. Overall, I thought Lost Era was better than Typhon Pact thus far). I thought it did a great job with Harriman and I also liked the Enterprise-B crew and what DRG III did with Demora Sulu. His take on the Romulans was great and Vokar was one of the best villains, especially among the Romulans, that I had read in Trek Lit.
I also thought he did a fine job with the Mission Gamma book. It was long, but it was well written and gave us a wealth of information about the DS9 relaunch characters.
It's a shame that I can't get into another DRGIII book since then. I couldn't stand Rough Beasts (despite the Romulan half-focus), Plagues of Night was just okay (saved by the cliffhanger ending), and I'm still slogging through Raise the Dawn.
All of the Typhon Pact books are well written in terms of details, though I haven't been a fan of DRG III's character choices. It's just something about the style that's just plodding, or too many unnecessary characters or something. Perhaps once I finish Raise the Dawn my opinion of the duopoly and Rough Beasts will change. But that remains to be seen.