To date there are two well known translations of Plotinus' work in circulation; Mackenna's and Armstrong's. Each translation contributes to the understanding of Plotinus' philosophy.
Armstrong's translations/editions are considered the cornerstone for modern scholarship. Armstrongs understanding of the Greek language allows him to get extremely close to what Plotinus is trying to say. His footnotes are especially rewarding and insightful.
A sound background in Plato and Aristotle is needed to understand Plotinus. However those who have had the ultimate mystical experince with 'Unity' itself are naturally excused.
If you desire to study and quote Plotinus, then it is safe to say that this is the 7 volume-set to have (so far).
As is typical for the Loeb classical library books, the volumes are physically small, and the original text (Greek, for Plotinus) is given on the left hand page, with the English translation on the right.
The Preface describes the historical context within which Plotinus wrote, offers a summary of this thought, and a survey of Plotinus translations, commentaries, and studies. This material is supplemented by short introductions and synopses at the start of each chapter, and by abundant and detailed footnotes. The footnotes explain translation difficulties (not uncommon with Plotinus), and also identify the sources of Plotinus' references to other writers. These materials are excellent.
The only thing that this edition lacks is an index. The editors plead the difficulty of indexing Plotinus, and recommend "Lexicon Plotinianum" by J. H. Sleeman and Gilbert Pollet as an alternative. This work is, however, out of print (is it even in English? I am not sure) so it is not a very helpful suggestion. As it is, given Plotinus' rather scattered way of writing, an index is missed.
The Enneads are a collection of Plotinus' writings from fairly late in his life. Porphyry, his student, encouraged him in writing down his teachings, and acted as his posthumous editor (he also wrote a short biography of Plotinus which is included in the first volume). The works as they exist today are as they were received from Porphyry. As editor, Porphyry created his own organization for the works based on subject matter. This order is completely different from the order in which Plotinus wrote them. Porphyry, however, did document the original ordering.
From my own experience, however, I would recommend strongly reading Plotinus' writings in the order Plotinus wrote them rather than the order in which Porphyry arranged them. The major advantage I found was that it was much easier to follow the reasons why Plotinus believed what he did, even if the subject matter does jump around a bit. I tried Porphyry's order first, and almost gave up in despair before trying again in Plotinus' order. I have come to the conclusion that much of Plotinus' reputation as a bad writer is due to unfortunate but well-intended editorial decisions by Porphyry. Given that the Loeb edition presents Plotinus' writings in Porphyry's order, and that the Loeb edition is in multiple volumes, reading Plotinus this way does have a certain entertaining quality as well (first get volume IV, read a treatise, then get volume VI, read another, then get volume I, read another, and so on).
An important recommendation I would make for the reader is that he be properly prepared in his background reading. All of Aristotle and all of Plato would be ideal (as well as a worthwhile activity in its own right), but if the would-be reader of Plotinus finds that a little daunting and wants to get started sooner, there are a few works that he should make a particular effort to read: Plato's "Phaedo", "Republic" (Books VI, VII), "Parmenides", and "Timaeus"; Aristotle's "Physics", "On the Heavens", "On the Soul", and "Metaphysics". Plato, as the earlier writer, should be read first (by the way - don't be discouraged when you find you don't understand the second half of "Parmenides", Plotinus is going to tell you what he thinks it means in due course, so all you need to do is understand the references). If you don't have Plato or Aristotle, for Plato, Cooper's "Plato: Complete Works" (in one volume), and for Aristotle, Barnes' "Complete Works of Aristotle" (in two volumes), are excellent.