I'd say Duane's TOS stuff is decidedly better than Dark Mirror (haven't read the DS9 novels at all.) Spock's World and My Enemy, My Ally both made a big impression on me, in particular. Basically, if you ever wanted some really toothy Vulcanoid ethnography novels with good TOS characterization, she delivers. I got the impression she simply wasn't as attached to the Next Gen characters. Which... fair enough, neither was I.