Express books are great for giving you the basic knowledge on key concepts/ideas/precedents within that particular legal area. However, they may fail to give you a real indepth anaylsis on certain things, for example... Express books may only provide enough information to give you a good answer and not a great answer (backed up by criticism, reform, dissenting judgements, and a balanced legal argument). However, they are cheap compared to normal books and have much positive feedback.
Goodluck.