My ex boyfriend was the product of his dad's second marriage, and for years believed that his dad had been separated from his first wife when he met his mum.
Turned out that it wasn't true, his mum was the OW and his dad did leave the first wife for her (although it may be true that the first marriage was over in all but name, I'm not sure).
But what was true was that ex MIL was the most bitter and unhappy person I've ever known - it was more than 10 years and 2 children into the relationship before they finally got married, and ex FIL was away a lot working in those initial years so I think she felt like she was the mistress who had been knocked up and then abandoned.
But then, by the time they finally married, I don't think there was much love lost between them and they spent years living separate lives under the same roof, hating each other. Worst of all was that ex MIL took it all out on the children of ex FIL's first marriage, refusing to acknowledge them as family or have them in her house. When one of them had children of their own, even the grandchildren were not accepted by her, even though the past family traumas were nothing to do with them!
As you can imagine the atmosphere in that family was extremely poisonous and her nasty attitude infected the relationship between me and my ex boyfriend too. He tried to deny that he was like her, but all his most annoying personality traits were the ones which reminded me most of her.
So, in that case, the OW got her man but was treated by him as a second class, inferior wife from the beginning, and she resented it and took it out on everyone around her, forever. They separated eventually, to the benefit of all, and I'm thankful she is no longer in my life.