Complete Champion works best if you're campaign is set in Greyhawk and/or prominently features the core 3.0/3.5e D&D gods. It's also the most DM-centered of the "Complete" books, which are generally thought of as extensions of the Player's Handbook (PHB). While there are new feats, prestige classes (11 of them, actually) and spells, the rules for creating church affiliations and holy sites take up quite a bit of the book. While players might join one of these organizations, it's the DM who'll get the most out of this information.
Affiliations were introduced in the Player's Handbook II (PHBII) as a way of providing rules and benefits for characters belonging to various organizations. You don't need the PHBII, however, as rules for building your own church affiliations are provided in the Complete Champion. Affiliations are useful because they provide criterion for characters to rise up in the ranks of a given organization (in this case, a church) and provide increasingly valuable benefits for doing so. While all the churches presented in the book are of non-evil Core/Greyhawk deities, there are brief descriptions for the evil Core/Greyhawk deities, as well. Furthermore, rules are given to allow you to create a church affiliation for nearly any deity you choose based on their prominent domains.
Among the usual selection of feats are a new type called Domain Feats. These feats, which can be taken by any character, are each based on one of the core domains from the PHB such as Air, Destruction and Travel. These Domain Feats provides an ability, often only usable once per day, based on the powers associated with the specific domain. This provides a fun, if not always balanced, way to provide non-divine classes with the blessing of a deity, which I like, but there's certainly nothing to keep a divine class from taking these feats as well.
Many of the prestige classes are based on the Core/Greyhawk deities, as well. Pelor has the Shadowstriker and the Shadowspy which are both tied to a Pelorian church affiliation called Pelor's Shadow Guard. While this might sound counterintuitive for a sun god, the description of the affiliation does a great job at explaining how these prestige classes came to be. There's also a prestige class for Hextor (Ordained Champion) and another (The Sancctified One) which can be taken by Ehlonna, Kord, Olidammara or Wee Jas, providing differing abilities for each.
The new spells favor the cleric, but there's quite a few new things for paladins, druids and blackguards as well. There's even a few new spells for Adepts (the NPC class), which is something I haven't seen in a while. On average, the spells are 2nd-4th level spells, but clerics receive a few 5th-7th level spells, too.
Of the new items presented I particularly enjoyed the special holy symbols. These are masterwork holy symbols that each have special powers relating to the Core/Greyhawk deity they represent. There's quite a bit of variety here and these items are relatively inexpensive having more value in terms of status than monetary worth.
The final chapter covers divine quests and holy sites. Again, this is probably more befitting a DM than a player. While players who worship a certain deity will certainly want to visit the holy site and gain the favor of their gods, this material is better served in the DMs hands. Still, these can add a fun touch to any Greyhawk campaign.
Overall, I think this is a great book and I intend on using the church affiliation rules to build my own churches and expand those of the other deities in my Greyhawk campaign. If you're running a 3.0/3.5e Greyhawk campaign or favor the Core deities, this book can definitely help mesh the rules and rewards of belonging to a church to the concepts and flavor of serving a deity. If you don't have any use for the Core/Greyhawk gods, then this book will be of significantly less value to you and I suspect that may be why it's received such low ratings overall. I'm sure I'd like it a lot less if it featured Forgotten Realms or Eberron deities, for example.
So, if you like the Core/Greyhawk deities and want to find ways to make them more interesting to their followers, pick up Complete Champion right away. Otherwise, you'll probably be disappointed. But me? I think it's quite good.