I would preface this with saying that Races of Eberron is NOT part of the Eberron line of books. It is part of the Races of XXXX books, in the Generic line. While the material here is still useful to Eberron players, there's a good deal here that you will already know. And, taken as a generic Races of XXXX book, its pretty good. It is by no means the next Draconomicon, but it is easily the best of the Races books (which, considering that lines contains what is easily the WORST book WotC has ever published, Races of Destiny, isn't saying much).
However, as an Eberron book, it falls rather flat.
Much of the material in the book is rather uninspired (the exception being the chapter on the Kalashtar, which was personally written by Keith Baker, the setting designer himself), and some of it is downright contradictory to the material provided by the Eberron Campaign Setting book.
The PrC provided are generally pretty good, with the exception of the ReForged (a PrC centered around a WF losing it's armor plating and becoming more alive, essentially an anti-juggernaught, but without the cool factor).
And, perhaps most dissappointing, is that the non-new races (the PHB races) are royally shafted. Keith Baker has a series of excellent FREE articles on Wizard of the Coast's website that go into much more detail than this book does. If you want information on how to play an elf in Eberron, this book is going to be virtually useless to you. And perhaps worse yet, it devotes as many pages, if not more, to the Drow (a race that was specifically said to be unsuitable for play in Eberron) as it does the other non-warforged/changling/shifter/kalashtar races.
The magic section is generally acceptable, although there is an entire line of artificer infusions that, as per the rules for infusions in the ECS, are not even usable as they target specific sections of a fleshie body, and not an item or a construct. Several items also seem to be oddly done (such as low price items with caster level prereqs of lvl 15-20, which as a rule do not exist in the setting).
If you are brand new to the setting, or would like to use the four new Eberronian races in another game, this is a book you will find useful. If you are an Eberron fan that is looking for the next Sharn: City of Towers, don't waste your money on this one. You'll likely find it to be mediocre at best, and flat out wrong at worst.
IMO, the only thing that gives this book any saving grace, other than the Kalashtar section, is that it was not made part of the official Eberron line, so it is easy to dismiss anything you find in it as such.