well then you are very lucky tigermoth! and you clearly didn't meet them with the same background and mental expectations as shopaholic has - "Old boyfriend mega successful barrister, rich, good looking etc. DH bit of a loser, bitter, angry and bad tempered...". I think that she is putting herself in a very risky situation...
I have an ongoing relationship with an ex boyfriend which is emotionally exhausting and ultimately probably damaging to each of our marriages (especially his as his wife has always been deeply jealous of my mere existence), but somehow neither of us can terminate contact. We went out 12 years ago, split up 10 years ago, didn't talk for about 3 years, and then started e-mailing, talking and meeting up. Long discussions about why we split up, raking over the relationship, complete honesty about how we felt about each other then, dawning realisation that we still love each other despite the fact that we are in good relationships with partners we love and have children with...... aurgh. So we are completely faithful to our partners physically, but mentally? Somewhat screwed up. We meet twice a year, in public, no alcohol, which is as much as we can take. And speak intermittently the rest of the time - sometimes 3 times a week, sometimes once every 6 weeks. It can be very unsettling, it can be very reassuring (aren't we lucky to have so many people who love us absolutely?) but I guess I wouldn't wish it on anyone else.
Bloody hell, that was a bit of a cathartic post!
So Tigermoth - don't do it. Especially if you feel your marriage is slightly ropey - that would have been fatal for me.