The base of Plotinus' philosophical system begins with what he calls The One, which is all things and no things... absolute unity, completely indivisible and set beyond existence. From The One emanates the less perfect universal Intellect from which the universal Soul emanates also less perfect, from which individual souls emanate. Plotinus postulates that every man is conflicted between a desire for individuality and a stronger, but poorly guided, yearning to return to the absolute unity of the one. He outlines that all beautiful things are more or less reflections of the Unity that all souls seek, but we are easily distracted by the reflections, blinded by the bodily, and led astray. he offers a cosmological view of the universe as it extends from the One and a partial guide to returning oneself to the One, although it is a journey he himself has not been able to complete.
Partially philosophic and partially a beautiful spiritual account, The Enneads are essential reading for anyone wanting to fully understand western philosophy; to see a crucial development on Platonic ideas and to see his influence in later philosophy/theology such as the works of Thomas Acquinas. It is so valuable its own right as a well written and thoughtful attempt to express something very familiar but unwordable that runs through the human psyche.
The Elmer O'Brien translation is a good introductory text for anyone wishing to become acquinted with, but not deeply familiar with the works of Plotinus. He presents a sort of "best of the treatises" arranged in a way that he finds most accesible to the reader. For the more devoted scholar, the multitude of Loeb copies will both be more accurate, more complete, more comprehensive and offer the oppurtunity to read the greek text directly, which offers many insights that can't be conveyed into a perspicacious english text. As an introductory read, however, the O'Brien far outweighs the McKenna translation in accuracy and conveys a tone somewhat more akin to the actual writings of Plotinus.

Few people today read Plotinus whose work ranked with Plato's and Aristotle's in Antiquity. Indeed a knowledge of this difficult and esoteric philosopher's thought is a must for understanding western philosophy through Spinoza. Unfortunately, MacKenna's edition-- the standard in English-- is lacking for many reasons (looseness and excessive liberty in translation for one). O'Brien avoids these pitfalls. This is a beautiful translation of a well-chosen representation of texts. Start with "On Beauty" for an easy introduction to a mystic tradition now largely forgotten. Make no mistake about it, however, Plotinus is difficult, albeit rewarding, to read under any circumstances (I almost suspect it is a proof of insanity to claim to comprehend fully "The Three Primal Hypostasis"). Luckily O'Brien has done several things to help the reader. Aside from a beautiful translation ably annotated, he supplies a useful introduction and an appendix of texts from Plato, Aristotle and the Stoics that Plotinus creatively appropriated and reinterpreted in a highly original way. O'Brien thoughtfully directs the reader's attention to the appropriate passage[s] in Plotinus. Read this book and you will begin to understand how the teaching of this esoteric Neo-Platonist was once a serious rival to Christianity. I highly recommend this book.