Let me just come out and say it right away: The ECG is the new yardstick by which every future 4e campaign setting will be measured.
Comparing it to the FRCG, It is far superior. It has way better maps, better organization, better art and better writing, and last but not least, it paints a much clearer picture of the setting. Now, do not get me wrong. It is still a campaign book that sets the tone, that provides the framework for the campaign, it is not a book that tells you what zombies Lady Vol prefers, or what High Cardinal Krozen eats for breakfast.
It does however provide the DM with some awesome tools and pointers to really make Eberron feel like Eberron. In fact, there is 11 pages of it, at the beginning of the book, where the authors go over the campaign themes (last war, draconic prophecy, dragonmarked, urban intrigue & dungeon delving) of Eberron and how to implement them in the campaign.
All this before they dive into the setting and start describing the different parts, from the national threats to the gods and passing by the countries and the 13 houses. I won't go into any details regarding the content, except to say that it definitely looks (for a non-expert such as myself) as if the new fluff of Eberron is very compatible with the older fluff. So if you have a host of old Eberron books, they should convert just fine - most of the time anyway.
As I started out by saying, this will be the new yardstick. Go buy this book, because frankly, it has everything a campaign book ought to have. And I am not even an Eberron fan. Or rather, I wasn't an Eberron fan. I still do not like warforged and shifters, but I would definitely play Eberron now. And I am most certainly going to yoink a bucketload of great ideas for my own campaign. Go buy this book, because even if you do not like Eberron, it is so full of great ideas that it is very hard for me to even imagine how it couldn't bring a lot of inspiration to any DM.
Rating:* * * * * *
Buy Eberron Campaign Guide: A 4th Edition D&D Supplement