I adored these when i was a kid in the 70s.
(On the strength of them, I also read his Star Wars tie-in from the same era, Splinter of the Mind's Eye. Liked that very much, too.)
I was able to pick them up a year or two ago, collected in 3 or 4 volumes. I did not read all of them – really I only read Yesteryear (still one of the best Star Trek episodes) and skimmed bits of a couple other stories. I should have also read Log 7, The Counter-Clock Incident, which is probably the one I loved most as a kid. Maybe I'll get around to it.
My impression is they do not hold up well.
It's worth noting, when you look at Foster's published work, that these are among his very first novels. So he was probably on a steep leaning curve as a novelist as he did these. The later "Log" books are probably much better than the first few. "Yesteryear" is in Log 1, which by my theory should be one of the worst of the books. So my impression may be hasty and unfair. I really, really adored these as a kid.
I read these in the Seventies as a kid also. I can still recall a couple of sentences verbatim after some 33 years:
"An infinitude of Kirks waiting for the return of a billion Spocks in a million variations of a certain awkward second or two in time..."
"Vulcans do not voluntarily pollute their bloodstreams with odd combinations of ethyl alcohol molecules."