I got the 4e ECS and the player's guide this week as well. I think that in terms of user-friendliness, these books are much easier to use for a quick and dirty campaign or just an adventure backdrop.
The main reason for that conclusion is that practically every entry (for an organization (ie. Aurum or a dragonmarked house) or a kingdom (ie. Aundair or Q'barra)) has an organizing NPC within it that kind of shows the flavor of each within it. I really liked how Mordain the Fleshweaver was written up actually. I plan to use his stats as inspiration for NPCs in my homebrew campaign.
Each geographic entry is also very useful as a one-stop information source. Each kingdom can pretty much be used singularly. The only other info you might need to reference are lightening rail linkages with other kingdoms and regions, as well as how international organizations like the Lords of Dust, the Aurum, or the dragonmarked houses fit in.
Overall I really like both books. As a comparison to the two 4e Forgotten Realms books, I think they're much more focused on playing an Eberron game than being a grabbag for DM's to use for their home settings. Its more unified in that regards, but that makes sense given Eberron's 'modern' sense of an internationalized interdependent political economy (ie. dragonmarked houses, airships, lightening rail, the Last War, etc).
My big criticism of the book was that it would mention all the cool magic-tech items and vehicles like the elemental airships, the lightening rail, etc. and it didn't have accompanying stats write-ups or sidebars for DM adjudication.
Sure I could go to the Adventurer's Vault and use the airship stats provided there, but I see the Eberron type magic-tech to be sufficiently unique that they would be given space. (And these aren't in the Player's Guide either).
Anyway, the reason I'm so interested in the magic-tech is because my home campaign uses a lot of airships and warforged/constructs and its partially inspired by Eberron, the Full Metal Alchemist, and the old Disney cartoon Tailspin. Follow the accompanying link if you want to see a lot of detail about that campaign and its world:
The Spire and the Abyss - home
Cheers,
C.I.D.