TDS is a sequel, but it starts with events even before the ones in TPM. Although this is not unique, it is not so common occurrence in fantasy books of this caliber. First part of book is told from point of view of Jardir. He didn't have much appearance in prequel, although his role had very impact. He was also described as something like a bad guy. Now we have simultaneously events from present (of the book), after the events in TPM, and from Jardir's past, which start few years before TPM (since he is older that Arlen, main character of TPM).
We are told about Jardir childhood and growing up in Fort Krasia, one of Free cities (I don't know why I though there are seven of them). Krasnians and their society were introduced in TPM, but here we have a much detailed and wider description of it. It is somewhat based on Islam culture: language, relationship between men and women, belief that death in fight guarantees you place in Heaven, etc. I must say that this is one of least likable societies I have read about. TPM wasn't one of the books that clearly differentiate people on good and bad: there were demons, who were the enemies, but it was their nature. Humans, the good guys, are not in any way all good, clean and decent: they are stupid, dirty, rapists, villains... So, when we got POV from Krasnian society, which was depicted somewhat badly in last book, I expected a turn, where they are shown to have redeeming qualities that are invisible from the outside. Surprisingly, this was not the case: even though Krasnians are not evil per se, or without any qualities, they are still described as stupid, wasteful, fanatic, without consideration and believing in their natural superiority. What is even more miserable is that this assessment is probably true for many societies in past, and even in present. And Jardir is a perfect specimen of his society. He believes himself as smart (and he is compared to some of his fellow citizen), but he doesn't even have the capability to understand how easily he is being manipulated. He did something awful to his best friend (actually, to many of this best friends), but even when he know that what he did was wrong, he excuse himself that is was his right and duty to do it. But in the end, he induces more pity than anger from reader.
His part of the book takes at third of the book, and it last until the events from his past start to concur with the present. Then we have addition of other characters, Leesha and Rojer, know to us from TPM. With them we have a direct continuation of events from the first book, an aftermath of battle of Cutter's Hollow, now known as Deliverer's Hollow. We have depiction of how everybody except two of them start to regards Arlen as new Deliverer, even though he denies it. We also have a closer description of politics in Free cities. There is one new and old character: Reena. She had a small part at the beginning of TPM; now she gets upgraded to a real main character. Interesting thing is that her plot doesn't have much importance in the course of the book. It is strange that author would invest so much in one pretty unimportant character, but her part was in no way dull. From one side it describes in detail life in one small part of the world, and from the other side goes deeply in characterization of few in no way ideal characters.
Things get much more intense later when Arlen gets his POV, after quite long time, and main characters start to mingle. It is always interesting to see the contrast between how character sees himself and how other characters see him. And we have a lot of this in TDS. Also, the clash of cultures is always fun to read. Arlen POVs are very interesting to read, not so much because of his plot, which is very intense in its own right, but because we have one character that we got very acquainted in first book, but then he got very much changed, and this change was seen only from third persons. He often remembers earlier Arlen and it very compelling when you can compare them, because they are in the same time very different and very similar.
All parts of book are very interesting and very fun to read. TPM was stretched over probably more than fifteen years; TDS, after retelling this whole period from point of a completely new character, concentrates on period of maybe three month. Even though the plot is interesting, it is action plot about clash of cultures, something familiar from other books or movies. What are unique to this book are its characters and their description from several different sources.
I also has to mention one more aspect. Even though the demon aspect of this setting doesn't have much coverage in this book, few short pages mean a lot. For first time we have point of view of a demon and we learn something about their way of thinking. This few passages succeeded in making me even more interested in this setting. Also, a nice touch was the indication that this is actually postapocalyptic setting in alternate future of Earth (books about war-machines).
Also, although I can't be sure without the reread of TPM, I think that Brett's writing style got improved in this book, which also adds to enjoyment of this book. Just a few technical details: book has around 750 pages. It also has a preview of the last book, called "The Daylight War", which is announced for 2012.
All in all, "The Desert Spear" is a very good book in its own right, and even better as sequel to "The Painted Man". Not dull in any moment, and with supreme characterization, it will be a good read to every reader who even remotely liked "The Painted Man". If you read the first book, don't miss this one.