I think it is of great use as a reference work for what's in it. As far as I can gather (as much in it is too modern to be my PRIMARY interest), it appears to be highly accurate.
My only issue with some of these "spotter's" books is that when you've got a vehicle which was, let us say, a 1972 conversion of a vehicle originally built in 1952, it will describe it and conclude "Introduced in 1972". OK, that's accurate, but could the building date not be added too, as in "Built 1952, Converted 1972", with a footnote at the start of the book to explain what the two dates signify?
That would be of immense use to researchers / historians, and modellers.
So that's my tuppence-worth.