I suppose everyone who uses Smalltalk should have a copy of this book, even if it's just to sit on the shelf, though you will of course get more repect if you have the older out-of-print version (S.80- The language and its implementation).
There are two problems with the book for a Smalltalk user of today:
1) The book was written a long time ago, and Smalltalk pioneered a lot of concepts in programming languages and IDEs, so very often the terms used for various things are not the terms we would use today; This makes for a confusing read at times.
2) The book is not very helpful from a practical point of view, if you are a new Smalltalk programmer struggling to get to grips with the class library because nearly every implementation these days has a plethora of non-standard extensions which obviously aren't covered, so what you are left with is a guide to the language syntax, which of course famously fits on one page of A4, and a brief look at some of the standard collection classes.
Bottom line: Buy this book to help Goldberg and Robson, because we owe them, read it, then go back to your implementation's help pages.