I do enjoy it when the last book of a trilogy turns out to be just as good as the first book. Middle books, eh. They can be long-winded or slightly boring or off-topic or without purpose. I mean, think about "The Two Towers." All that riding around Rohan and trekking through the woods with Ents. Snore-o-rama. And then, blam! The last book nails it.
It'd be silly to equate this trilogy with Tolkein's, but my point is the same. Scalzi returns to the first character he created, John Perry, brings along his now-wife, Jane, and the adopted daughter, Zoe. Plants them in a strange setting and puts the fate of the world on Perry's shoulders. Many plot twists later, and you have an excellent set-up for the end of the book.
Plus, he adds a wise-cracking secretary who makes you hoot with laughter. He should be a humor writer, really, this stuff doesn't just roll off the keyboard. Although, amusingly, this character makes it clear that he does write a (very well-read) blog on the side (http://whatever.scalzi.com/). Because writing a blog is all about the clever.