Essential Cell Biology has excellent diagrams (consistently labelled and coloured throughout the text), a good selection of photographs, and clear and readable explanations. It has the common apparatus of textbooks (an opening summary and a list of keywords for each chapter, and a glossary), but the outstanding pedagogical feature is the set of questions which accompany each chapter. Great effort has been taken to make these interesting and thought-provoking, and full answers to all of them (not just the numerical ones) are provided. A minor complaint is the absence of any references or further reading suggestions: simple pointers to the relevant sections of Molecular Biology of the Cell would have been easy but useful.
It is obviously intended as a university or senior high school text, but Essential Cell Biology is so well put together for self-study that I recommend it to anyone interested in the subject. It is non-repetitive, conceptually rigorous, and doesn't cut any corners, so it could happily be read by academics from other disciplines — but at the same time it is a book which could inspire a bright 12 year old. Writing for readers without any background knowledge is actually harder than writing for graduates, so I think Essential Cell Biology is actually a greater achievement than Molecular Biology of the Cell.