I have been using them for close to a year. They are excellent for most things and really make cleanup easy however you need to be careful if you intend to cut food in them they are basically thin plastic. When we make chip dip or sauces they are nice because we can pull the liner out and dump the dip into a bowl or container easily. Overall I wouldn't cook without them again. You can get store brand WAY cheaper than the name brand which helps. We buy the Meijer brand liners.
We add them later, but normally we are cooking on Hi for a few/several hours instead of low overnight, and yes, we have disintegrated them before.
I never use high that might be why I don't have them fall apart. The best is chili on low for a at least 15 hours absalutly amazing!!!!