The Western church has embraced the first part of Athanasius' formulation, while largely ignoring the second. The notion that man might become God is viewed as apostasy in many quarters. Yet you have to consider both parts to get a complete understanding of incarnation. If the incarnation has to do with Jesus alone, we might be tempted to dismiss it as some sort of cosmic stunt -- a neat trick perhaps, but so what? On the other hand, if we have it in our nature to become God, then incarnation must be taken seriously.
Jews at the temple in Jerusalem prepared to stone Jesus for blasphemy after he declared, "I and the Father are one." But then Jesus turned the tables on them, quoting one of the Psalms: "Is it not written in your law, 'I said, you are gods'?"
I think Athanasius can be challenged on at least one point. Strictly speaking, it is not accurate to say man becomes God, since this suggests the initiative belongs with mankind rather than with God. No spiritual adept that I know of has ever suggested our divine nature can be realized by anything other than God's grace. The operant word is realization, since our true nature is always divine. Technically, we can't become what we already are.
How does man becoming God differ from God becoming man? Ultimately, I think the difference is largely semantic. If the initiative remains with God, then the movement is always from God to man, whether we're talking about Jesus or anyone else. Jesus didn't become God; God became Jesus. And so it is with us. In truth, perhaps the only thing you can say about incarnation is that God becomes more fully himself.