I actually enjoyed these quite a bit. I think they are best in the three Star Trek: The Classic Episodes collections which actually bundle the stories in order by season. If I remember correctly, part of the reason the earlier ones strayed a bit was because James Blish had to work from scripts and wasn't always able to watch the episodes. Since some of the scripts were changed during filming, this created some discrepancy. Because the works were so short, he didn't intentionally expend any of them.
One interesting thing to note, his wife believes Harlan Ellison may have been the person who talked him into taking on the tasks of adapting the episodes.