When your ex finds a new love it can bring mixed emotions. Depending on the circumstances, they may have already begun this relationship before they became your ex of course, but even if they managed to keep it in their pants for a week or two after walking out of the front door it can be quite a strange thing for you to deal with at first. (Although it should hardly come as a surprise that they instantly swap your bedsheets for someone else's when your guitarist ex's favourite joke of all time is 'What do you call a musician without a girlfriend? Homeless!')
My advice is to resist the temptation to drag the nearest person off the street and swing them from the chandelier in a blatant act of revenge sex. This may make a strangers day a bit more 'special' than they imagined when they woke up that morning, especially if they'd just popped out to buy a newspaper(!), but in the longrun you'll just feel cheap and nasty.
Just sit back and feel smug knowing that once the 'honeymoon period' is over someone else is eventually going to have to deal with all of the challenging stuff which you went through. You can also take great pleasure in the fact that if she was remotely involved in his mysterious vanishing acts during his relationship with you it'll only be a matter of time before he's playing Secret Squirrel once again with somebody else - a leopard never changes it's spots. Simply count your blessings that it's no longer YOUR problem.