It is also very useful for paladins, but I think clerics are going to get the most bang for their buck out of it. At the very beginning of the book there is a section entitled Playing An Effective Cleric . The 3rd Edition has really changed the effectiveness of the cleric, and this section tells you how to take advantage of all the new rules. Of course, the next section is entitled Playing An Effective Paladin . I didn't find this section as useful.
Paladins are pretty straight forward in their strengths and weaknesses.
This also includes all new paladin mounts and special mounts for those with the Leadership feat. Let's talk feats. There are fourteen new feats presented in this book tailor made for divine spellcasters. If you are like me, you probably realized early on that the Extra Turning feat was cool, but not that cool. Many of these new feats have Extra Turning as a prerequisite, and they benefit from you having that feat as well.