I like magic, and Eberron is a world that represents an archetypical high-magic world. Keith Baker, who wrote the Eberron sourcebook (with help) and developed the world concept, also wrote Complete Arcane. Those two facts led me to have high hopes for this release, Magic of Eberron. What I got was something of a mixed bag, in my opinion.
What you get:
As promised, there's more information on the Shards, elemental binding, grafting different things onto your body, info on Daelkyr, Quori, Karrnath, and Aerenal magic. The typical run of feats, PrC's, spells, items and creatures including two new races (half-Daelkyr and psiforged). Someone also took the Dragon Totem feat and ran with it, with a number of new related feats.
What you don't get:
Any information connecting the info in Complete Arcane to this book or Eberron. Magic in any of the realms other than Karrnath, Aerenal, and Zilargo (except how the gnomes use binding).
The PrC's and many of the feats deal with each of the above items. So there's a PrC each for binding elementals to yourself, fighting Quori, Karrns v. undead, protecting Aerenal, and so on. 6 or 8 all together (sorry, don't have the book in front of me). A couple of these PrCs seem to turn character into what their fighting, to some extent. For example, the PrC for Karrns that fight the undead presence in their land gradually becomes more like undead, even to being able to get turned/rebuked.
The feats are mostly related to Dragon totems, Draconic Prophesy, grafting parts onto people, and psionics (specifically relating to Quori). The spells, powers, creatures and magic items were generally related to these same themes as well. There were some new things that didn't relate directly to a theme of the book, but not many.
In all, there are 158 pages, and I thought it could have been improved with another dozen or so pages. Specifically, I think they could have added more information about how the different cultures use magic, where things from the C.A. fit in, and relating more information back to the larger world. Really, it's the usual $30 for smaller book by comparison to many of the other resource books we've been offered.
If this were a movie instead of a book, I'd say, yeah, see it, but wait until it came out on DVD. Should you buy it? If you are running a game, and looking for more hooks or unusual things to bring to the game, sure. As a player, borrow one for a bit to see if the themes interest you. If not, don't waste your money. I gave 4 stars because the book does relate to the world, there are consistent themes, and things tie nicely together. It did not get the fifth star because I would have liked to see at least some connection to the C.A. info, as well as more cultural details around magic, and it was too short a book.