I bought these for my daughter who is almost five and in Reception year at school. She loves this set already (we've only had them a few days!) and can't wait to read them.
Each book focuses on a different sound, eg 'short e' or 'plurals', and all the words within the story relate to those sounds, which helps to re-inforce the concepts.
The inside front cover is really good for parents, because it lists words to look out for in the story, other 'key' words, and other more complicated ones. I think this shows that they are really well thought out. My daughter loves to spot these words on the way through, and anything that expands her vocab and renews her interest in reading is fine by me. Many of the words used are part of her 'keyword' learning at school so she thinks it's great that she can read quite a lot of the story by herself, and it's a real confidence boost for her.
All in all, well worth the money.