I bought these for my daughter in a charity shop initially.
Although they are suggested for 12m+, she loved them from 7m. At first she just held them and bashed them together, but something about them (colour? shape? I could never work out what) seemed to particularly appeal to her, and she favoured them over all her other toys.
Now she is 14m, and still loves them just as much, and has graduated to squeaking the chicks, and fitting the eggs together with their shells.
I can see they will continue to grow in play potential with her, as eventually she will figure out the bases of the eggs need to be 'sorted' into the box, and later still that the faces on the shells can be matched to the chicks. I have to say her 9year old cousin is always volunteers to 'sort' them for her, just to be helpful, you understand!
I noticed any other babies or toddlers who were visiting seemsed to be very keen on them, and have consequently given them as gifts on birthdays etc.
I really dont know what it is about the eggs, but babies seem quite transfixed by this particular toy.
I would highly recommend it for any baby or small child, it has lots play potential, which changes as the child grows and develops new skills. It represents eggsellent (sorry, couldn't resist that) value for money.
It is also very compact, a great toy to stick in your bag when you are heading out on a visit, as it holds their attention for ages.