I use these while toilet training my child. I use these instead of pull-ups because I find these allow the children to feel the wetness and pull-ups don't. They sell plastic covers that go over these to block leakage as well.
WE used these when potty training my oldest, and I'm not sure I would use them again. I don't really see the need for them since they still leak like undies. Plus they were really tight in the thighs for her as well so she didn't' want to wear them.