This is the kind of book that receives a four or five, first for the place it holds historically within the field of its study. After Prolegomena, study of the Torah would never be the same. Thankfully, my Ph.D. studies, in an evangelical seminary, esteemed the importance of our being exposed to the best of biblical scholarship, and, so, this was required reading.
Certainly, the volume reflects a tendency toward a rationalism alienated, many would say, from the devotional attitude essential to commune rightly with sacred texts. Regardless, what Wellhausen, and other biblical critics of that era, especially of German descent, offered is a contrast to the assumption that sacred scriptures should remain free from the rigorous textual and historical study applied to other works. I see this as a move in the direction of integrity, so cherish much this volume and the whole of the biblical scholarship like to it of that era.
I would say this kind of volume is for serious students of the field of scholarship pertaining to the tome. This is not an easy read, certainly not an inspirational read, unless one is inspired by in-depth scholarly criticism of a historical-textual nature. Reading Prolegomena is like going on a textual archeological dig, slowly plodding along, layer upon layer, tell upon tell. Also, like such landmark works, this is somewhat out of sync with present-day scholarship, but classics themselves never truly get out-of-date. Prolegomena, being a classic, is timeless and deserving of a read by being such.