The authors are pretty against line extension in the simple sense of slapping a big brand name with "equity" on other products just to make them look good in the light of the big brand. They argue that a company stands for a word in the mind of the consumer, and the first one to grab a word will own it forever as long as they maintain their focus. So the right way to expand is through dedicated brand names per product. This was a nice wake-up call and harkened to a lot of similar ideas about leadership and winning a whole market segment from Zero to One.
The book's examples are somewhat dated (referring to computer technology as very nascent and speaking about Lotus as the leader of a category), but that didn't detract at all from the main points (in fact, it was eery evidence of how our predictions can be wrong in either direction, where some things change a lot while most things stay the same).
Overall, I really enjoyed the book for its new perspective. It's interesting how so many of its laws are violated all the time, and I wonder if the authors are right about the long-term consequences for those who do violate them.