I've never felt the need to put them on my bikes, but when my kids were young and riding goathead infested fields and trails, I put them on their bikes pretty quick. Didn't eliminate flats but reduced them by about 75%, which was pretty good considering I was fixing flats just about daily.
Personally, I'd look at tougher tires before liners. If you are still having lots of flats, then go to the liners.