Talon of Tiamat
[spoiler]Logical Entry: Any class capable of getting +4 BAB by 8th level. Anyone with an Int score to invest CC ranks 4 skills, and willing to burn a feat and a starting language (on a relatively common language by CO standards).
Cons: Half casting (on a class that doesn't even require casting, thus implying the PrC is intended for Wizards or some such), Medium BAB (on a PrC that has a BAB requirement), limited use Breath Weapon (though the damage on it scales somewhat nicely, and a feat can be taken to make it once/1d4 rounds instead of once/day/breath weapon type). Minor bonuses to Bluff (easily outpaced), a really crappy Fear aura (limited by HD), and every major ability is Cha-based (including a 1/day Dominate Dragon). You're better off using Totemist, Dragonfire Adept, or even a Dragon Shaman or Sorcerer over this class.
Pros: The Breath Weapon is actually fairly potent if you take the right feat (14d8 Fire every 1d4 rounds). Dominate Dragons, just because. EX Immunity to one energy type.[/spoiler]
Apostle of Peace
[spoiler]Logical Entry: Any Spellcaster. It has a high Will requirement, a Concentration requirement, and requires 4 feats (two of which are actually eating your feat slots).
Cons: Aside from everything that's wrong with Vow of Peace as is? The fact that prior to Magic of Incarnum, Tome of Battle, or Tome of Magic, the skill requirements could only be met by a 7th level Cleric, Warlock, or Spellthief. Maybe Ranger or Pally. Oh, and the contradictory phrase that lets you use Magic Items while still maintaining VoP. A limited spell list hurts too.
Fact: The bad side of this class is both mechanical and social, as this class imposes some strong restrictions on your allies and you alike. Be prepared for some headaches people.
Pros: Turn Undead, Censor Outsider, and 9th level Divine spells in one class.[/spoiler]