I know this is one of Dr. Norman Geisler's favorite apologetic arguments; he finds it particularly compelling. It probably would be to medieval or modern minds. To the post-modern mind, however, it sounds like you just pulled some sort of logical trick. This is why this argument is rarely seen outside of academic apologetics.
Personally, my only objection to the ontological proof is that it assumes that existence is greater than non-existence. That makes sense to me, but you have to prove that existence is greater than some sort of Nirvana-like non-existence. That's not easily done.
Personally, I prefer the ethical argument (natural law) and the cosmological ("uncaused cause") arguments. I find them more compelling.
No, you simply have to ask if there is anyone in the audience prepared to argue that non-existence is superior to existence by putting a gun to their head and pulling the trigger.
That your opponent continues not to commit suicide is empirical evidence he thinks existance is superior, no matter what verbal arguments he may rehearse for the gallery.
In fact, it's quite impossible without a whole host of tendentious assumptions.
I own, have read, and constantly refer to Dr. Geisler's Come, Let us Reason. He represents a current in the Reformed Church that has rediscovered Thomism and cautiously endorse it, within their denominational constraints. In spite of that, I find that after 500 years of Protestantism, many Reformed thinkers have come to appreciate the value of good, classical, traditional Christian theology and philosophy--which so happens to be Catholic.
-Theo