The material in this book is so well organized that it provides flexibility to be used in both undergraduate and graduate level. Some additions to the previous edition of the book, like expanded coverage on multi-rate signal processing and discussion of the cosine transform, made the book a great up-to-date resource for the people studying signal processing.
I'm sure that every one in engineering, dealt with signal processing, already knows Oppenheim's reputation. That leaves nothing to be told about the book.