Reading it now, I was more interested to identify a political agenda than I had been in 1979. But L'Engle escapes easy categorisation here (my memory is that the later books in the series are a bit more didactic). The hellish world of Camazotz inflicts equality through conformity on its inhabitants; a naïve reader might see this as a commentary on Communism as perceived in the hottest period of the Cold War, but I see it as equally applicable to 1950s America (which is in fact the particular hell to which the Camazotzians are condemned). It's also notable that the motives of the US government (strictly, Meg's father's employers at Cape Canaveral) are not questioned at all. L'Engle preaches individualism but also a loving compassion for others; this is how Meg defeats IT. She also of course has angelic beings sweeping in to help, but with limitations; they provide transport and guidance, but Meg has to find her own way in the end.
Anyway, a book that deserves its reputation and well repays re-reading.