This book is a masterpiece. It includes a critical text, and a line-by-line philological commentary. But even the reader without Greek will learn an enormous amount about Plato and related topics by reading it alongside a translation -- just skip all the entries dealing with purely philological matters.
It is often said that the best commentary on Aristotle is Aristotle. Hence, important commentaries on Aristotle spend most of their time quoting (in Greek) other passages from Aristotle. The same is true for Plato - and probably for all philosophers. So keep a copy of the translated works handy and whenever Dodds or anyone cites a passage or refers to a passage, follow up the reference.
The best translation of the collected works remains E. Hamilton, Cairns Lord -- not Cooper. By a mile.