Both the introduction and the translation by Hugh Lawson-Tancred are very much below the standard one expects from Penguin. Most importantly, the English of the translation is frequently incomprehensible. I advise everyone who wants to study this masterpiece of a book to use another edition, e.g. that of George Kennedy, published by Oxford in 1991, or the Loeb edition. That is, if you want to understand why so many people in so many ages found this book brilliant!
As a trial lawyer and a pragmatist, I've long dismissed philosophy as the useless art of contemplating one's navel. That assessment began to change recently when I audited a continuing legal education seminar in which the speaker analyzed trial advocacy on the model of Aristotle's "Rhetoric." His speech was brief and his analysis superficial, but he'd aroused my curiosity. I got this book and read it.
The general principles Aristotle formulated for forensic rhetoric over 2,000 years ago still hold true in the 21st century courtroom. Some of the specifics have changed (e.g. no torture for slave witnesses), but human nature hasn't, and human persuasion hasn't, either. Aristotle's "Rhetoric" should be required reading for all first year law students. I regret not reading it 30 years ago. Apparently philosphers do more than just stare at their navels.