To Atheists: Ontological Argument -- What do you think of it?

I'm not posting to spark a debate, I just want to hear what atheists have to say about the ontological argument. Just for the sheer dialectical fun of it, no debate intended.

For those of you who don't know, this is (just one format of) the argument:

I. It is possible that a maximally great being exists

II. If it is possible that a maximally great being exists, then that being exists in some possible world

III. If a maximally great being exists in some possible world, it exists in all worlds (or else it would not be maximally great)

IV. If a maximally great being exists in all worlds, then it exists in the real world.

V. Therefore, a maximally great being exists.

VI. Therefore, God exists.

Just wondering where you guys think this falls short, which premises dont proceed from the previous and/or if any assumptions are questionable. Thanks :D

EDIT Since many of you are confused about what a "possible world" is, let me clarify. It is not some parallel universe or any other such nonsense, it is a conceptual world that exists merely in speculation. For example, unicorns exist in a possible world by the mere act of me picturing unicorns existing.