I did a OneWorld redemption to Spain and Argentina around Thanksgiving. I was on American to London then connected to Barcelona on British Airways (have to use two airlines besides American), took Iberia from Madrid to Buenos Aires and American back to New York. The Iberia and American Business class experiences were very similar, in my opinion, with maybe a better wine/food selection from Iberia.
Even better, though, I just got back from Spain on a cheaply booked coach fare -- Iberia to Madrid and American back. The turista (coach) class was full going and they gave me a complimentary upgrade to business plus. Considering my ticket cost just over $500 roundtrip, that was quite a bargain