It's a good menswear reference manual for time period it covers - basically, the 20th century prior to 1972. It's certainly not perfect. Its current day relevance is in many ways limited - since many things really have changed since the mid-20th century. Even within the time and place with which it concerns itself, it's not always unbiased, nor always correct.
I say this on the basis of having examined a copy, years back.
If I found a copy at a yard sale, priced at $5, sure, I'd buy it. But if I found a copy being sold on eBay or Amazon for hundreds of dollars, thanks, but no thanks.
Ah, it just occurred to me, you know what I'd liken it to, at least in a way? The edition I have, from the 1930's, of Emily Post's, "Etiquette." Well worth reading for the insights it provides on its place and time, but certainly not a bible in the sense of it being completely relevant to the modern day.