Fr. Hardon, I am glad to say, is a model of orthodoxy. This book is well-organized, and the sections on abortion and the Virgin Mary are particularly good. The writing, however, is very wordy and sometimes ornate; after 570 pages, it gets to be a bit much. Worse is his tendency to dismiss serious objections to the faith with little more than hand-waving, saying in effect, "Now let me just inform you of a few facts and your objection will vanish." For an overview of the basics, though, it is reliable.
I remember, more than 20 years after the fact, reading this book cover to cover while in college, and realizing for the first time that the whole thing made deep sense. I have recommended this book as a basic adult introduction to the teachings of the Catholic Church many times since, and continue to do so today.