In short: The first half of the book (which was originally published as a novella in 1953) is brilliant science-fiction—the work of a true Grandmaster. The second half...not so much. The ending is ambiguous to a fault and felt rushed, as if Blish lost control of the themes he was trying to develop and settled for wrapping everything up quickly.

50 years later is it relevant? Yes and is a worthy read among Hugo winners. Mainly due to it's rare marriage of Catholic theology and the dilemma of discovering faithless alien life. The sociological themes were a mess of Cold War fanaticism and 1950's Mad Men style male dominance.