I did not first read this book in my youth, years ago, so there is no nostalgia to color my reactions now, and I feel mild contempt for this book. It's not just that it's slapdash and unsatisfying, either; there are many things wrong here. Though published in 1971, this book reads as if written in 1951: it was written during the Apollo Moon landings, after the film and book "2001: A Space Odyssey," and 6 years following the publication of "Dune," yet there is no hint that Blish noticed any of these.
The pacing is uneven; tangential digressions give way to leaps of years in the main story line. The author seems to know (or care) little about ships, space, planetary ecology, evolution, or unmanned probes. Characters are barely two-dimensional, yet manage to be annoying nonetheless. The gender relations are fraught, as though Blish was angry at the Women's Rights movement around him and wanted to say so, but not fraught in a way to serve the story. Blish tosses around techno-babble as if to delight a 13 year old nerd, but in terrible dialogue it becomes onerous. A sample for you: "'...there is no accompanying mass effect, and having found that out from theory in advance, you must have assumed that the contraction equation was meaningless under the conditions of your drive-field. I made that assumption too, but with the evidence now in hand, I can see where the error lies.' (p. 86)" People on a ship's bridge during a crisis, saying that? Nah.
As a matter of fact, it seems Blish just didn't care. Either this really was a 20 year old manuscript he sent off to satisfy a contractual obligation, or he just didn't care what he fed the market at that point. I don't see any reason I should either; to paraphrase Pangborn's Davy, just call it 'spinach' and to hell with it.