In writing certain articles on this subject, my opinion regarding the soundness of the ontological argument tends to be one that is violently swayed from time to time; rejecting it on grounds of logical unsoundness to even accepting it and arguing for it by considering some inadequate objection and showing it to be an illegitimate criticism. However, I think at this point I might be grounded in my position that I reject Anselm’s proof, once and for all.
However, that rejection doesn’t necessitate the argument’s lack of worth for natural theology. For instance, Anthony Kenny (2004) happens to consider Anselm’s argument valuable for discussions of religious language. My personal mentor and friend, rejects the argument as a viable proof for God’s existence but likewise considers it valuable for natural theology. Particularly, God understood to be the Greatest Possible Being (hereafter, GPB) gives us a very insightful understanding as to the attributes and character of God. As he once wrote:
From the definition of God, one can infer what attributes a GPB would necessarily have whether he actually exists or not [ … ] This being would, as well, be infinite in duration (if this being existed this being would have to always have existed, as, if this being was limited in duration something could than be greater). This being would also have to be unable to be negatively effected by anything outside of himself (I use the gender reference out of convenience, not as descriptive as God has no gender), infinitely creative, true and the source of truth (meaning minimally, the standard by which all good things are set or to which they are compared), simple (meaning not being divisible or positively being a completely unified being), and not dependent on anything else for existence (not being caused by another). All these attributes extend from a understanding of the definition. This is not an exhaustive list but it illustrates how the attributes are inferred by the definition. 
Although I would tend to agree with him, we unfortunately differ on how we would go about rejecting the argument. Since, him being a Thomist, he would reject the argument on the grounds that we do not know being or essence directly; i.e., we know essence through effects. Hence, this is why causal arguments are legitimate. I would side more so with the camp that existential propositions don’t entail logical necessity. In other words, the proposition “God exists” being an existential proposition by no means entails that the person this proposition denotes is by any means logically necessary.
Philosopher Mortimer J. Adler I think got it right on this point when he rejected this argument as a viable proof for God’s existence once we consider the affiliation with existential propositions this argument has. As he writes:
No existential proposition can be self-evidently true or necessarily true. When we have direct perceptual acquaintance with a particular individual, the existence of that individual is evident to us, but the proposition which asserts that the individual exists is not self-evident, nor is its truth necessary. 
My criticism can be extended to say that we have a gap between where we are and where God is. This doesn’t mean that there is some gap in our knowledge that leaves us and God out of reach (per se), but that the mere phenomenon of our conception of God by no means entails his actual existence. However, God as Anselm understood him, is still very valuable for the scope of natural theology and its own understanding of God.
You as the reader might accept the argument and find the argument perhaps more than valuable as an argument for God’s existence, or maybe you just aren’t sure for yourself. You might side with Plantinga when he suggests that “it establishes, not the truth of theism, but its rational acceptability. And hence it accomplishes at least one of that aims of the tradition of natural theology.”  You may also side with certain philosophers who find the argument to be just a word game, worthy of no real merit. Whatever you may think, I do hope that the relevant function of the argument’s understanding of God is something that impacts your thinking regarding God’s existence and his attributes as a necessary being.
-  Taken from Faith|Reasons (FR2G); the post can be found here.
-  Mortimer Adler, How To Think About God (Touchstone: 1980) p. 104
-  Quoted from Graham Oppy, “Natural Theology” in Alvin Plantinga, ed. Deane-Peter Baker (Cambridge University Press: 2007) p. 22