What do you think of this argument for God?

[Nonpareil]

I'm addressing this to you, because you do seem interested in actually discussing the argument. There are a number of atheist and also unfortunately Christians who argue it, and clearly don't even understand the syntax being used.  For instance up until recently, I largely ignored it, because I thought that possible worlds was speculation about the multiverse.

There are people who treat it as that, yes. Plantinga is one of them. I obviously disagree, as do most others who make use of modal logic; a "possible world" is simply an abstract concept, a logically coherent way that the universe "might" have been. It does not possess any sort of existence beyond this.

This is just clarification, by the way; I'm not intimating that I think you would disagree with this.

First I was curious about your referenceing ipse dixit in regards to premise 3  and citing Plantinga as conceding this.   I would just like to see it in context.

I mentioned this in my first in-depth dissection post, but for full reference:

Alvin Plantinga is a Christian apologist (I don't say this to disparage him, just to make it plain that this is the explicit basis that he uses for all of his positions) with a particular interest in the various ontological arguments. In 1974, he authored the version of the modal ontological argument that I showed in my previous post - the one that makes use of the S5 axiom set to define God as necessary. He published this argument in his book The Nature of Necessity, wherein it was presented as a reformulation of Saint Anselm's original argument.

In the conclusion of that same book (page 221, specifically, which can be found on Google Books if you're curious), he stated:

"Our verdict on these reformulated versions of St. Anselm's argument must be as follows. They cannot, perhaps, be said to prove or establish their conclusion. But since it is rational to accept their central premise, they do show that it is rational to accept that conclusion. And perhaps that is all that can be expected of any such argument."

The "central premise" that he refers to is that it is possible for a being with maximal greatness to exist. And, as I have pointed out, while Plantinga asserts that acceptance of this premise is rational, he cannot actually show how or why this would be so. As such, the premise is bare assertion (it is his attitude towards it being bare assertion, his attempt to simply tell people to accept it anyway, even though he knows it's faulty, that leads me to use the label "ipse dixit" here). And, again as even Plantinga himself admits, this is the best that can ever be expected from any version of the ontological argument.

And this is entirely without getting into all of the other potential issues with the argument, such as incoherent definitions (as "goodness" is subjective) and misuse of axioms. As I mentioned previously, while S5 is valid, it is not the only set of axioms that modal logic can make use of, and each set of axioms has very specific limitations and applicability. Modal logic is not my particular specialty, so I haven't taken the time to sit down and work out whether or not Plantinga should have been working under S4 or another set of axioms instead, but I suspect that he might have - and, under those other sets, he would not be able to pull the sleight-of-hand trick with "possibly necessary" that his argument rests on.

But, as I said, I haven't actually verified that personally, because it doesn't really matter. Whether or not S5 applies, the central premise of the argument remains bare assertion, by Plantinga's own admission.

As such, the argument is discarded.

It uses the modal form of the argument that I have normally heard used

Yes. That is Plantinga's argument, slightly rephrased to put the language of modal logic in layman's terms. You can swap the phrases "exists in every possible world" and "is necessary" without changing anything, mechanically speaking.

Epistemically, one may say it is possible God does exist and possible he does not. However, we are interested in metaphysical possibility. It is on these grounds one must object. That objection can come in two forms: A. The concept of a maximally great being is incoherent,[2] or B. We do not (and/or cannot) know whether or not the concept of a maximally great being is metaphysically possible.

Correct, but with the slight quibble that it is entirely possible for both of these objections to be true.

It is also demonstrable that the latter objection is true; we have no idea whether or not the existence of a god is possible. That is why the argument fails to establish its conclusion. It is precisely the premise that both Plantinga and myself have pointed out as being unsupported. If we knew that the existence of this necessary being were possible, we really wouldn't need to have this discussion - but since the concept cannot even be coherently defined, let alone be established as possible, the argument is a non-starter.

Rather, it establishes that God’s existence is either necessary or impossible. Either the maximally great being is possible or impossible. This is why it is such a great argument!

I'm not sure I would call it a great argument for establishing that the concept of God includes the fact that he is necessary. An important definition, maybe, and something that might influence someone's thoughts on God, but the argument itself is just kind of dead on arrival.

I have also heard William Lane Craig mention a similar two conclusions to the argument

As a general rule, I would discourage listening to anything that William Lane Craig has to say. He's an idiot.

And I don't mean to say that he is an idiot because he is a theist. Plantinga is a theist, and while his formulation of the ontological argument fails to establish its conclusion, he is at least an intelligent man who can build a coherent argument within the laws of modal logic. He understands how prepositional calculus works, and how to employ sets of axioms, and so on. He is an intelligent man who just happens to let his beliefs color his thinking a bit farther than is advisable (and, admittedly, doesn't understand the theory of evolution and mouths off about it more than he really should). I don't agree with much of what he says, but he does at least have some legitimate grounding as a philosopher, and he is at least honest about the failings in his formulation of the ontological argument.

Craig is an idiot of the highest caliber. I am honestly surprised that he possesses the necessary brain function to walk and chew gum at the same time. His one and only talent is as a bullshit artist. Do not be fooled by the "Doctor" in front of his name; while he does technically possess a Ph.D. in Philosophy, this is only because the institute that he went to did not offer doctorate degrees for anything more specific. He has always and only ever studied philosophy of religion, and without any actual critical eye turned towards any of it.

Again, I'm not just saying this to try and be dismissive (I mean, obviously I'm dismissing Craig, but you get what I mean). If you actually want to learn anything about philosophy, logic, or really anything at all, you would be best served by ignoring Craig and everything that he says or does. Even if he were an atheist, and arguing against the existence of God, I would say the same thing. The man simply has zero understanding of logic as a system, and any argument that he advances can be safely thrown into the garbage bin without a second glance, as it is guaranteed to be full of more holes than a sieve.

Anyway. I'm more than happy to carry on this discussion, though I admit to being slightly hazy on what particular part of the quoted section you wanted my thoughts on. I picked out what appeared to be the most important points and gave a brief rundown of each, but if I missed anything, just let me know.