I have to disagree on Plotinus. He's easily one of the most underrated philosophers around, not one of the most overrated.
The problem with Plotinus is that modern scholarship on Plotinus is terrible, for a number of reasons.
1. Most Plotinus scholars are continentals and analytics...neither of whom have any business doing the history of classical or medieval philosophy. Or, more often than not, philosophy at all. But I digress.
Case in point: Anyone here familiar with Lloyd P. Gerson? In his book "Plotinus: Arguments of the Philosophers," he actually makes the claim that there is an argument for hypostatic Intellect from the existence of eternal truth.
I am almost certain that he pulled this out of thin air.
An analytic historian of philosophy is someone who thinks that if he plays linguistic and logic games with a text, and failing that, tries to "reduce it" to common sense and common language, he'll end up with a correct or plausible reading.
Anyone familiar with Peter Geach? Analytical philosopher who wrote books on Aquinas? No? Good. Don't bother. Not worth it.
Fact is, if you're not someone who's knee deep in the Aristotelian and Platonic traditions, you're just not going to understand Plotinus. If you don't have, not only a good knowledge of the Posterior Analytics, the Metaphysics and at least some of the Physics...well...let's just say that's the bare minimum, and if that's all that you're bringing to the table, you still won't understand him.
Lloyd P. Gerson is famous for his exposition of the "two acts" theory in Plotinus' thought.
He's famous for that. That's a "big discovery" in modern scholarship.
Oy vae.
2. We only have access to fragments of Plotinus' recent predecessors among the middle Platonists, and we have absolutely no writings from his teacher, Ammonias Saccas.
3. The commentary that Proclus, a later Neoplatonist, wrote on the Enneads has since been lost. And as far as I'm aware, we don't have any other ancient commentaries on the Enneads.
From 2 and 3, note carefully: Just because we are alive at a later date doesn't mean that our understanding of an historical period, of an historical figure, of a philosophy, etc. is somehow better than those people of an age gone by. Texts get lost. Ideas fall out of the contemporary zeitgeist. Things get forgotten.
4.I think that Armstrong is just right on this point: the Enneads are an unsystematic presentation of a systematic philosophy. In addition to the problem of the difficulty and obscurity of Plotinus' thought, which is DEEPLY and COMPLEXLY scholastic, in addition to the fact that we have him, so to speak, "ripped out" of his historical context, that he appears in the midst of a veritable sea of forgotten and lost personalities...
...in addition to all of that, he's just downright obscure, and more on some occassions than on others. To quote Kevin Corrigan: "Plotinus never says the same thing twice." He's not like other thinkers where he will copy/paste and expand basically the exact same thing in different contexts when he's talking about the same thing. He's not like Kant where he will basically repeat himself ad nauseam so that you basically know exactly what he's going to say next.
The dude's writing is both dialectical and obscure. He'll entertain 12 different positions at once (a bit of an exaggeration, but not by much), argue through them all, and you'll be left scratching your head wondering what he actually thinks. And then just wait until he takes up the same topic a few treatises down the line!
He simply lacks the expositional clarity of a figure like Proclus.
But...
...
All in all, he's definitely a Platonist, and one of the most important ones. Proclus and Plotinus are probably two of the most influential Platonists in the middle ages and decidedly helped shape the course of Arabic and Western scholastic thought. Avicenna and Al-Farabi both knew a version of Plotinus and Proclus, albeit through arabic paraphrases, integrated them into their own systems, and passed that on to the Christian scholastic west.