I personally feel that anyone who attempts to use the Ontological argument in a serious debate should be hit over the head with a barstool.
Anybody who thinks Descartes is a Philosopher we should listen to, other than his famous 'I think therefore I am', then they should be hit over the head with a barstool too. The Ontological argument is...well...stupid, I even said that when we looked at that in religious studies 2 years ago, everyone, except one dude in the class agreed, the dude of course was one of those people who wasn't exactly intelligent, said 'how is it?' Shaking my head I told him what was wrong with it.
Though I think Kant pwnd Descartes and his version of the Ontological argument better than any of us could.
"The definition of God is that he is perfect and that's how we imagine him, but for something to be perfect it HAS to exist, so if we're imagining a perfect being, therefore it exists." It doesn't take a genius to know that's rubbish. Descartes didn't because he meditated in an oven and put a question mark next to everything people talked about.