Anyone have any experience with sequestrum surgery on their horse? 

Yes. If they need it, they need it, otherwise you'll have a chronic open wound that can become infected.  Usually its a very simple surgery and recovery time is quick. One of mine even was done under local anesthesia, while standing. 

yea, mine had surgery to remove it, never missed a beat and never lame. Still running strong now.

Yes, my horse fractured his mandible and ended up with a bone sequestrum.  They had to go in twice to remove pieces.  They have to be dealt with or you end up with an open wound that will not heal.  The last time they removed pieces it was pretty much closed up and healed in 4 weeks.