The Complete Divine is a must-have for anyone who enjoys playing a cleric, paladin, or their divine counterparts (druids, blackguards, etc.) -- and is eminently recommended for anyone "stuck" playing a "Party Medic" who wants to jazz things up a bit.
THE GOOD: The new core classes are excellent. Of particular interest is the Favored Soul core class, which is the clerical equivalent of the sorcerer (and an updated version of the "Evangelist" class concept that first appeared in Dragon Magazine some months ago). Many important 3.5 revisions of prestige classes that first appeared in other sourcebooks can be found herein (from sources such as: "Defenders of the Faith," "Masters of the Wild," "Faiths & Pantheons," and even Dragon Magazine). If those aren't enough, there are several new entries as well, fitting a dizzying variety of character concept molds. To help with this, the book designers even organized a list including each class in terms of the strengths they service (good or bad characters, strong spellcasters, strong physical combat characters, etc.). Feats abound for every divine class. With several new or revised Wild feats, druids are given myriad new options that more than make up for the lack of prestige classes available to them (which better supplements the strengths of playing the druid class than prestige classes can to begin with, anyway). New and revised Divine feats round out cleric- and paladin-based characters, and there is a decent collection of General and Metamagic feats available as well. Particularly impressive is the introduction of the concept of relics -- items that require an element of sacrifice from those that wield them (usually a divine spell slot of a given level, worship of a particular deity, or both) but which grant more bang for the player's buck than "standard" items. An expanded overview of character death explores options and expectations one might be curious about should that dreaded natural "1" come up on your saving throw.
THE BAD: Frankly, there are more prestige classes than you will ever use -- even if you play divine spellcasters of all faiths and persuasions from now until Armageddon -- and for all that, as others have noted, Druids get the shaft in terms of prestige classes anyway. Besides the section on relics, only a few crummy new staff concepts appear in the Magic Items section. General information is included on a multitude of deities -- making selecting a god for your character to worship a less confining task -- but only for the core world of Greyhawk. The table of monster deities detailing "What worships Whom" is interesting, but don't look for any detailed information beyond the table itself because you won't find anything. A brief overview of church organizations and theocracies is given, but it's no more (if not less) detailed than the information originally presented in the old "Defenders of the Faith" splatbook. Finally, this book caters primarily to players -- not DMs. DM's are best-served if they're running an NPC-heavy game and are looking for options with which to pit the players against "unique" opponents.
Overall, be sure you buy the book because you enjoy having options in your game (and lots of them). You're almost certain to impress your fellow gamers when you whip out a new power they've never seen before, and you'll definitely keep your DM and fellow players on their toes the next time they face one of your "run of the mill" clerics.