I worked with some Mexican and Cuban women and they could communicate on one level but there was alot of pauses and you could almost see them reaching out for words that the other would understand.

Then they snickered at each other when the other walked away and both groups would say: "They don't know how to speak Spanish!"
I'd have to think that pure Castilian Spanish would have alot of differences from the Western Hemisphere styles just due to the Indians involvement.