I did it a couple times. One of them was a real good friend of mine for a few years. I never saw her as somebody I would fuck, but that's because I was getting steady poon then. Then I hit a dry spell, and one day we were texting each other shit back and forth and she sent me "fuck you" and I replied with "please do," in which she followed up with "really?"
Then it struck me. When I thought about it, I would fuck this woman. I didn't even remember the husband until she said she was going to get a hotel for a couple nights and pay for it with his card. I thought that was a real shitty thing to do, but then I thought about all the sex that I was going to have and I just really didn't care. It had been a couple months and I was ready to get mine.
The funny thing was the age differences between all of us. I was 24 at the time. She was 35, and her husband was 52 (I think). He got so angry when he found out about it. Another little twist was that one of my friends was fucking her for about a year. When we started fucking, there was a good 6 months before he found out about it. It's not like I tried to keep it from him. I mean, he was fucking other people, too, but at one point they started talking about marriage and how she was going to leave her husband. Of course, it never happened. My friend took it the wrong way. He didn't get mad at me, but he thought if he started fucking her again (she quit for a little while when she thought we were going to become an item) that I would feel the sting he felt. Needless to say I didn't care when she went back to him, even though she tried to keep it a secret, but he would tell me and apathy would just wash over me.
I didn't really care for all the drama that surrounded it. In fact, I only fucked her every couple weeks or so. It was just often enough to keep me sane, and not often enough to draw any suspicion from other people.
I'll tell you what. There's nothing hotter than somebody drunk out of their mind sending you a text asking you to come over and fuck them, and then being able to leave with no strings attached.
EDIT - It ended when the husband found out that my friend got her pregnant, and that she got an abortion. It just got too weird for me at that point. The dude came over his house with his son wanting to start shit (which is also kind of funny because the husband's son is about the same age my friend was).