I just finished re-reading Bantam's Star Trek: The New Voyages, the first professionally published Trek anthology from back in 1976, and one of the earliest books of original ST prose fiction ever published (only Mission to Horatius and Spock Must Die! preceded it). It's also probably the oldest surviving book in my personal Trek collection (I do have a couple that are physically older but that I haven't owned as long), and it's been ages since I actually read it. I read the stories so many times back in the early years that I still found them very familiar, but it was interesting to revisit them with a fresh perspective and see how well they held up.
The highlight of the volume is still the concluding story, "Mind-Sifter" by Shirley S. Maiewski. In concept it's a bit derivative of "City on the Edge of Forever" and "The Paradise Syndrome," but it's still pretty effective and emotionally engaging, and it's striking what a large and ambitious story it tells, spanning nearly 2 years of story time. (Although it has a couple of plot holes by virtue of being so focused on the personal consequences of its events that it glosses over the larger ramifications.) Another highlight is "Visit to a Weird Planet Revisited" by Ruth Berman, in which Shatner, Nimoy, and Kelley are beamed aboard the real Enterprise and have to play their parts like never before (the flipside to a previous fanfic story, "Visit to a Weird Planet," which can be found online). It's a bit implausible how quickly the characters accept the incredible situation, but it's still rather fun. I also think "Intersection Point" by Juanita Coulson holds up very well; it feels like it could've been an episode of the show. Maybe not one of the best episodes, but it does capture that '60s-TV flavor very well.
The other stories are kind of a mixed bag, and a couple suffer from feeling too derivative of specific episodes. And while they're probably rather sedate for stories that started out as fanfic, there is one story ("The Enchanted Pool" by Marcia Ericson) that arguably falls into the "Mary Sue" category, and one or two that have a vague hint of slashiness. (And "Mind-Sifter" does somewhat fall into the classic "hurt-comfort" fanfic category.) Overall, though, the anthology does a pretty good job feeling like Star Trek rather than indulging in the excesses fanfic can be known for.
I don't still own The New Voyages 2, so I won't be moving on to that. I remember it being less satisfying overall, which is why I didn't keep it.