Based upon my own experience, One World is strictly for the benefit of the airlines, not the customer. I purchased tickets from American Airlines to Singapore which included flights on both AA and JAL. My wife flew before me, and American wanted to check her bags only through to Narita, where she would have to claim them and recheck them to Singapore. Only after talking to several personnel at AA, did they check them on through. Getting assigned seats on the other airline is a mess. JAL does not know what to do with an AA confirmation number, and AA will not book a seat for a JAL flight (or, I found later, an AA flight on a JAL airliner). Even when you have the JAL confirmation number, if it is an American Airlines flight on a JAL airliner, your goose is cooked as far as getting an assigned seat--neither airline will do a thing: JAL says it cannot book because it is an AA flight, and AA says to call JAL because it is on a JAL aircraft. One World, no thanks!