There is only one reason to read "On the Origin of Species" -- to discover how Darwin himself first articulated the most revolutionary scientific theory of all time. And to achieve this purpose there is only one means -- to read his original argument, set forth with the greatest force, clarity, and brevity in that very first edition published in 1859. So, unless you happen to have the $$$ to buy an actual first edition, this facsimile of the first edition is the *only* way to read Darwin: all other paperback "Origins" publish Darwin's latest edition.
But even if you are not interested in the history of biology (scoundrel!), and you think you'll learn complete evolutionary theory from the "Origin" (fool!), you should get this edition -- and *not* later ones. Darwin's later editions of the "Origin" contain many errors that are not found in the original edition, including especially a progressive weakening of his original argument (evolution by natural selection) by the importation of Lamarckism (evolution by the inheritance of acquired characters). In these later editions, Darwin had been convinced by blockhead, mystical *physicists* that his *geology* was wrong (as if!), so he had to speed up the timing of everything, which meant smuggling in Lamarckism.
Last, this volume contains an introduction from one of the most charming biologists and philosophers of all time -- Ernst Mayr. This intro alone is worth the price of the book.