It would have been a sufficient introduction for the 1980s (for the revised Penguin edition) however Hibbard's Michelangelo has aged, and not too well.
Hibbard's analysis and overview of Michelangelo's art is acceptable, he does not say anything radical or particularly original but it is well founded. Some of the anecdotes on Michelangelo's life are fun, however it does distract as Hibbard is not very clear as to how it factors in with regards to the artwork he is presently analysing.
One aspect I really took issue with, considering it was the 1980s, the fact that racist terminology from Hibbard (particularly on the Last Judgement fresco) was not removed for the reprint - it does not lessen the original scholarship in terms of purely formal analysis of artworks, but it certainly dates it and shows a political passivity from Hibbard.
Equally on Michelangelo's passionate poetry directed towards male patrons he 'loved', Hibbard is suspiciously ambivalent and disregarding on this issue; but given, this was the 1980s, and should not excuse it. These two factors are why I cannot give it above 2 stars, otherwise it would be 3.
I was recommended this by a lecturer, as "still the best general introduction to Michelangelo". This is simply not the case. It may have been in the 80s or 70s, but studies on Michelangelo have progressed significantly as have popular biographies - incidentally, this book struggles to decide which of these it wants to be.
Nevertheless, it is worth reading. I would not cite it as a definitive work on Michelangelo's art, nor on the Renaissance, but it would be useful for someone looking for more dated scholarship; itself now part of history.