If you can assimilate Zimmerman's "Percentage System" AKA "EPS" it can be very freeing. If you are the kind of person who gets an idea of her own and can't be easily put off it, and you knit, you owe it to yourself to learn her method. But if you go into a yarn store with something in mind and the clerk puts you onto a pattern and it's always okay with you, I would say you don't need it. But the EPS can liberate you from needing a pattern for everything.
She is an opinionated woman, as she admits by calling herself, "The Opinionated Knitter". To get a flavor for her personality, read "Knitting in the Round" or watch the DVD "Elizabeth Zimmerman's Knitting Glossary", in which her daughter interviews her. Her daughter is Meg Swansen, an awesome knitter in her own right. EZ established Schoolhouse Press and M. Swansen runs it now.