This guide is what I would call the ultimate in a Player's Guide for D&D 4E. I did not use Eberron much in 3rd edition, we only briefly used it towards the end of our time with 3rd. I am currently running a game in 4E that is set in Eberron, but I have been basing all of the material off items from the 3rd edition.
With this guide, this helps hash out all of your PCs history. This has information on each different area of the world, and helps you write out your backstory (which I'm now requiring my PCs to do). Each area has a description of that part of the world, and lists the major features of each. It also has information how to adapt your character to fall under one of the Dragonmarked houses, and other scenarios from the history of Eberron. The book does introduce 3 new races (Changelings (shapeshifters), Kalashtar (dark fugutive humanoids), and Warforged (constructs)), and one new Class, the Artificer. It also has lots of new paragon paths, and a few epic destinies, along with a new skill, Alchemy, which is similar to and can be replaced by Rituals. And as with all player books, there are lots of new feats, and equipment, dragonmarked, normal, and magical. And the book has information on Genasi, which I did not allow one of my players to use since I was holding out for more information on Eberron, I am giving her the option of using this (with the limitations that are in the book of course).
My biggest complaint is similar to the main Eberron campaign setting from 3rd edition, lack of information on the world outside of Khorvaire (the main continent). I like to throw my party into a wild world, and there are barely 4 pages describing the rest of the world. Hopefully this will be somewhat remedied by the Campaign setting book due next month, but I have little hope.
As I stated previously, this is a very great guide for players (and DM's) to help develop their characters and establish a great backstory for them. I highly recommend it to players who are in the Eberron module.