We discovered this cup when our second child was having problems gulping from a spouted cup or bottle causing her to choke and vomit. A Doidy cup was suggested and it immediately solved the problem, as she could suddenly see what she was doing.
With our third child we have used them since starting him on solids, and they are also much easier for the parent or caregiver who is feeding the child to see what they are doing. I even hear good reports of this being used to top a newborn up with milk whilst breastfeeding is still establishing. A fantastic, if simple, idea. And they come in lovely jolly colours too; my son is particularly fond of his pink one!