Have had one flip (to me) so far. America was in huge unrest (started having rebels overrunning their empire, which was weakened from a war with Germany) and Boston flipped to me late in the game while I was going for a Cultural Victory. Unfortunately, it was near useless thanks to 22 turns of unrest, so I gave it away to Bismarck for Open Borders, so I could flood his country with musical tours. Overall, it's a "meh" feature. It doesn't happen often, you may not get border cities (meaning you might get some isolated city in the middle of an enemy's empire) and the huge unrest waiting period (which makes no sense, since they're supposed to WANT to join me) makes it pretty pointless.