I give the author an A- for the ideas, and a D for the execution.
There is a core here that might make epic play reasonable. By introducing quintessence, it draws a line between gaining XPs for killing mortal things and something else for killing gods. And, yes, you can kill gods, but it's mostly only possible if you're one, too.
The various ranks of deities is comprehensive, and the ways in which they connect to the universe as they become more powerful are effective. Other games tell you that your character has become a god and no longer cares about the material world. This book explains why they no longer care about the material world, and what they care about instead.
It introduces portfolios, which are essentially the next step for cleric's domains. Here I think there could have been more creativity in the various powers associated with each portfolio. They really run together and seem kind of the same. At the Lesser Deity level, every portfolio gets a high level Effect ability. At the Hero-Deity level, they all get a big competence bonus to to some set of skills or abilities under some set of circumstances. The portfolios provided really lack flavor, though it wouldn't take a lot of work to toss some powers out and install ones that do have flavor.
The last chapter starts by providing more than 200 new (or kind of new) epic feats. I don't really like all of them, and I'm not sure that the system the author has come up with to replace Increased Spell Capacity and other epic metamagic feats works nearly as well as he thinks it does, but it's worth trying out. Still, there has been a desperate need for more, and better, epic feats. This is followed by the abilities that deities get at various levels. They work like Super Epic Feats (and Ultra Epic Feats, Ultimate Epic Feats and Ridiculously Epic Feats). On the whole, I like them quite a bit.
Now for the problems. This book is horribly confusing to read and try to figure out. The rules for gaining quintessence, particularly through Glory and Worshipers, are complex and confusing. I still haven't managed to sort them out. The chapter on deific ranks is notably lacking in anything describing the process for moving up the ranks. If you gain enough quintessence, do you automatically go up to the next rank, or are there Greater Deities with more than 120 Hit Dice? If it's the former, what happens if there isn't any place in the cosmos for a new Sidereal, who are the personifications of planar layers, entire planes, or even dimensions. If a Greater God goes to 121 Hit Dice, does that mean that a new planar layer is created? Don't look for the answer in the book, because you won't find it. At least, I haven't yet.
There are two problems with the chapter on feats and abilities. The first is that there are places that are just poorly written. On page 147 there is a Siphoning [Effect] ability. When you use it, do you simply cause damage that subtracts from the amount of quintessence your opponent has, as is implied by the way that the ability is written up, or do you steal that quintessence and add it to your own, as is implied by the example given for the ability? There are a number of these sorts of glitches. Potentially a bigger problem is that there are a large number of abilities that refer to non-existent products. Some of the abilities provide a new template for your character that it says will be "detailed within a later Immortals Handbook supplement." Given the extent to which this company specializes in vaporware (Hey, when is Godsend coming out?) that is absolutely inexcusable.
On the whole, I really feel like the author treated me with contempt by doing such a poor job of basic editing and selling me a product that is incomplete. So, while I can recommend picking it up if you want to strip mine it for ideas for really high level gaming, I dislike the idea of putting money in the author's pocket.