They are a godsend if you have a traumatised nipple.
BUT....

1) they are a PITA cause you have to clean them all the time, and carry them with you.
2) Yes they sit on easily to start with, but as soon as bubs pulls off, they come too. So if you have a child like my man tonight, you would be repositioning them 10-15 times each side!!
3) They reduce sensation, thus lower milk produiction.
4) They look awful when breastfeeding in public. Very un subtle and undignified
5) Can cause nipple confusion like dummies or bottle teats.

Thats it as far as I am concerned... but like I said - they were a life saver!!