I'm a first-year physics student, and I found this book to be really helpful, but not as a main textbook. I used Grant&Phillips' Electromagnetism as a main book, from which I learnt the concepts, but often I looked up the proofs in Duffin, as they were usually clearer and simpler. (eg Dipole E-field) I bought this before Grant&Phillips, and I found this book a bit confusing and overwhelming.
One thing which is especially good with this book, though, is the end-of-chapter Comments, which explain some of the more interesting, out-of-syllabus things which you always want to ask but never remember to...