Well, when I saw this was coming out I was tempted not to buy it. I thought to myself, "What is this going to say that Lords of Darkness hasn't already said in a fairly definitive way?" Then I thought, "Well, the WotC stuff coming out has been pretty good lately, so give it a whirl." So I bought it. My first thought was the better one.
For any Forgotten Realms aficionado looking for 3E info on "evil", go to Lords of Darkness. OK, it's not 3.5 compliant, but that doesn't really matter as the interesting stuff is the text (evil organisations and so on) not the stats. Champions of Ruin doesn't really bring anything much more to the party that is terribly useful.
The main gimmick with CoR is that it suggests how to run an "evil" campaign: i.e. the PCs are evil and out to get the good guys. This is hardly a purely FR concern: you could run an evil campaign in virtually any gameworld. In a sense, this is my beef with this book: it probably would have been a lot better if they had made it "generic", as opposed to FR specific. Then they could have made it more of a sourcebook for designing evil campaigns, how evil might work in opposition to good and how evil characters could work in concert with each other, plus different orgnisations showing different evil motivations, and so on. Instead, it falls between two stools, and has to add something to the FR universe which has basically already been said by Lords of Darkness.
The book has the following chapters: an intro on "what is evil?" (i.e. what motivates evil characters to be evil); a races section with two new races (the krinth, a sort of darkness creature bred by the shades, and the extaminar, a "half-yuan ti" with much more limited powers; both are ECL +0, and neither especially compelling) and a monster level progression for the draegloth; a "tools of evil" section with new feats, spells and magic items; prestige classes; evil organisations (this is either something of a rehash from Lords of Darkness, or covers some rather obscure organisations you can either take or leave); evil places (shrines, nodes [the new favorite, along with touchstone sites, to pad out books] and the like); sample evil NPCs (not un-fun, though hardly amazing); and "champions of evil" (some epic level evil nasties).
A few words on the above. The tools of evil section isn't bad. Some of the feats are quite good (including some new vile feats, and initiate feats for some deities not covered in the Players Guide to Faerun). Also, some of the new spells are fun, and it gives some epic level spells too. The prestige classes are hardly core, tying in with some pretty obscure organisations and not really adding much that others couldn't (like the Justice of Weald and Woe, a very long title for an elven bow expert that works for the elven supremacists of the Eldreth Veluuthra and could easily be ignored in favour of the arcane archer). And the epic level champions of evil are a fun read (some are pretty gross, like the Elf Eater) but how many of us actually run epic level adventures?
Overall, the book seems to lack a coherent message about what it is actually trying to do. Frankly, it has its moments. But if you own Lords of Darkness, you don't need this. And if you don't own Lords of Darkness, buy that instead of this.
It's a shame. I thought WotC had given up putting out sloppy knock-off cash-ins like this. Well, I was wrong.