I'm just going to jump right in and get this out of the way early since I know you're expecting it anyway: place this book on your shelf right next to George R. R. Martin's series A Song of Ice and Fire and let the best series win.
On the one hand, the comparison is almost inevitable. First consider the high-level view: if you built a graph with Size of Tome on one axis and Particular Variety of Fantasy on the other, both series' markers would be drawn on exactly the same spot. They're both massive works; they both focus heavily on the Swords aspect of swords-and-sorcery and trend into the Sorcery part only as the work progresses; they're both heavy on world- and character-building. Plus both authors demonstrate a propensity for killing off significant characters in almost haphazard fashions. Even the characters and events in the books are remarkably similar: both focus on the destruction of a powerful family and follow the orphaned children as they are left to their various fates in different parts of the world; both involve an unexpected invasion from the ice-covered, uncivilized lands; both wind back and forth between military adventures and political intrigue. So from a sheer construction point of view, the works are remarkably similar.
On the other hand, most fantasy readers would probably agree that Martin's Song of Ice and Fire is a landmark work, sitting head and shoulders above every other sword-and-sorcery series available today. So even after subtracting points from Durham for lack of originality, you have to be impressed that he's managed to produce something that can sit comfortably on the same shelf. Whether Acacia can stand on its own is almost irrelevant: it's standing in the company of giants, so even if it's favoring a gimp leg it's still in the right company.
With all that in mind, I waffled a lot as I tried to decide how to rate this one. Durham offers detail aplenty, taking his time and really trying to seat the reader firmly inside his world. His motion through the plot is excellent: he focuses on key scenes and plays them out well, then advances the plot quickly to the next significant event so that you're spending your time visiting only the important parts. But for all that, Acacia still manages to plod from time to time--as if there were too much detail, or maybe that it's invested in the wrong things. And even that's probably not a fair criticism: several times he returned late in the game to some previous event and made its importance suddenly obvious, even though at the time you might have found yourself skimming sentences because you weren't sure why you should pay attention for the next ten pages.
Durham also has trouble making his work feel as gritty and realistic as Martin's. When Martin describes being caught in a cold snap, with steel becoming brittle and fingers turning black with frostbite, you can almost feel the crunch of ice-crusted snow underfoot. When Durham puts us in a sweltering desert we understand that it's hot, but it's just never as visceral.
Ultimately, after reading this book and pondering my review, I made up my mind about two things. First: yes, I will probably buy the sequel and read it--which is after all the best compliment you can give an author. And second: I need to find A Game of Thrones and re-read it, because Acacia has whetted my appetite for Martin's masterpiece.