My opinions here...
Sharn: City of Towers - 5 out of 5 If you want to go for the urban Eberron noir feel, than this book is worth its weight in gold.
Races of Eberron - 2 out of 5 - Some nice fluff, but also some inconsistencies and errors. I've used a few of the warforged feats in this book, but otherwise nothing.
Five Nations - 3 out of 5 - Very good if you want information on the five nations that made up Galifar. Mechanically, it isn't so great, but good fluff.
Explorer's Handbood - 4 out of 5 - I liked it, others didn't. I think it has great information on the airships, elemental galleons, and lightning rails. Also it has some very neat locations around the globe for adventure sites.
Player's Guide to Eberron 4 out of 5 - This isn't actually for players, IMO. Good information but also some inconsistencies. A DM can take selected articles and pass them on to players, but too many secrets are blown in this book for it to really be a player's book.
Secrets of Xen'drik --- I haven't looked into it yet. My campaign isn't heading to Xen'drik any time soon, so I haven't spent the time to read this.
Dragonmarked - 4 out of 5 - Good stuff! It has great information on the Houses and some neat feats for Dragonmarked characters. A few of the PrCs are pointless, but overall it is a good book.
I hoped this helped.
EDIT: I missed two.
Faiths of Eberron - 3 out of 5 - Has some good information, but I found most of the articles to be stretched out too long. They said something about the faith, and then repeated it for three pages. It is a good book and does help, but it is low priority compared to the others.
Magic of Eberron - 2 out of 5 - I had to go back and look at this book to even remember what was in it...that says something. It is a bunch of filler options that just don't fit anywhere else. I have used exactly one monster from this book in my game, nothing else. Skip it.