Let's share our ideas about coursebooks. Referring to the 7 comments, this is my opinion:
1- Of course it's impossible to find the perfect book. It's 4 years now working with Incredible English Kit (Oxford University Press) and, although it's a very interesting and motivating coursebook, I must say that I sometimes need to search for extra material such as songs, videos, stories... that complete my lessons.
2- I work with very young learners and experience has shown me that a good coursebook must have themes related to the child's world. If lots of recycling and pictures are added too, then it's much better.
3- Using a Student's Book, a Workbook and a Teacher's Book is not enough. A teacher needs other type of materials, such as flashcards, realia, storybooks, puppets, etc. This supplementary material may help reaching the didactic aims.
4- The English Department at my school chose the coursebook. The English teachers have the last word in the matter.
5- I prefer coursebooks with central characters and a story. Actually, IEK fits pretty well. And children really enjoy, as they get familiar with the main chraracters of the coursebook.
6- The workbook my students use doesn't have an answer key. Otherwise, they could just copy the answers.
7- I make all my own materials for the 4/5-year-olds. First graders use a coursebook (with no workbook), which is mainly used for oral work.