Paul Graham and his editor(s) are excellent. His prose is light and easy to follow. The only awkward component of the book's organization is that he tends to use a concept one section before explicitly introducing and defining that concept. I'm not sure yet if this is a good or bad thing.
Among books recommended to potential Lispers, ANSI Common Lisp is typically written off. Graham's style of Lisp is called "non-idiomatic". That's fair, both ANSI Common Lisp and On Lisp feature aspects of Common Lisp that lend themselves to functional programming. And as those of you who've read Practical Common Lisp know, Common Lisp (unlike Scheme) was not designed to be a functional programming language. Ultimately ANSI Common Lisp covers the same topics Practical Common Lisp does, if not more. But ANSI Common Lisp is better written, in less space, and with shorter examples.
I'm impressed at Graham's ability to summarize. There is a graphic illustrating symbols as a structure composed of a name, a value, a function, a package, and a property list. Although other resources (books and otherwise) mention symbols as having one or more of these components, his graphic was the first representation that clicked for me. He also provides clarity about packages being namespaces for names (symbols) not objects or functions.
And toward the end of the book, there is a discussion on the "instance" abstraction (relative to the class definitions themselves) being more powerful than plain "objects" that carry around methods themselves. This has been the single most useful discussion on the implementation of object-oriented constructs I've read yet.
Practical Common Lisp is often called the best introduction to Common Lisp. After reading both, I'd give Practical Common Lisp second place or call it a tie. The issue with Practical Common Lisp is that it takes too long to get anywhere and the practical chapters themselves are just as much a slog. And for as big as it is, Practical Common Lisp still doesn't include some major (potentially confusing) aspects of "modern" Common Lisp like ASDF, Quicklisp, production deployment strategies, etc.
Even after having read Practical Common Lisp I wasn't really clear how to pull together all the libraries I needed to get anything real done (e.g. scripting against an HTTP API or interacting with a SQL database). This is not to say that Practical Common Lisp is a bad book, it is a good book. But I definitely don't recommend reading it without also reading ANSI Common Lisp. And regardless, there are still a few of those modern concepts neither book covers.