What lies beyond Athas' Tablelands? What is in the centre of the Sea of Silt? And what exactly does the Dragon get up to in his considerable spare time? The Valley of Dust and Fire answers many of these questions and then some.
In a nutshell, this expansion book is partly a campaign expansion, and partly an adventure. It describes in great detail the regions beyond the East of the Tablelands and delves in to the Dragon's (secret) home city of Ur Draxa.
The book commences by providing some good descriptions of the very harsh physical conditions present in and around the silt sea, including the Grey Death. Some tables on weather and wind provide a good distraction for folks wanting to randomise these things on for adventuring beyond the city state of Balic (for example). And the descriptions of the various types of terrain are excellent.
But the utility of the book slows down thereafter. This is simply due to the fact that there isn't sufficient motivation for a party of adventurers to go much further in to the sea of silt. It is a truly harsh wasteland lacking in vital resources (like water and food) and filled with deadly creatures (that may or may not be so edible) and physical conditions that make the slave pens of Tyr or Urik seem appealing. The creatures that inhabit these lands are described in the volume (some are duplicated in the Monstrous Manual for Dark Sun). And moreover, the book also describes the silt skimmers (and other methods of transport over the silt's expanse) in detail which is good for any campaign that wanders near this area (but see also Guistenal later on…).
The book then delves in to specific locations in and around the sea of silt: an island beset with tornados, the Vanishing Lake and others. These are neat and could readily be transplanted to other settings since the memes themselves are solid, if a little specific.
After that though, we start to get deeper in to the domain of the Dragon: the Valley of Dust and Fire itself. Why the PCs would venture in to a lava ring around Ur Draxa is questionable at best. Hence from herein, the book is basically a campaign expansion with little reason for the PCs to ever visit. With the Dragon being the (un-killable?) predator of the whole campaign, heading to the Valley and Ur Draxa should be a one way ticket. Perhaps use it as a grand finale in a dubious attempt to slay the beast? Good transportable portions include descriptions of the burning plains of lava, dead forest and the smoking lands. Excellent fantasy stuff (in a post-apocalyptic sense!).
The detail of the Valley and Ur Draxa (a.k.a. the City of Doom!) itself is fantastic though. Ranging from the deathly politics of the nobel houses through to the patrols that guard it against any incursions, the city itself is interesting, isolationistic and xenophobic. Maps of the centre of the city are provided along with some points of interest that might plausibly be of limited interest to other campaigns. The domain/sanctum of the Dragon himself is also given, should the PCs consider slaying him. Which they won't because he's a super-intelligent beast who's been living for thousands of years and is a psionic / magical monster who plans a lot of contingencies in-spite of his prowess.
Three scenarios for campaigning in the Valley are also given: a Visitation, an Entrapment (oh please no: not the you're all slaves meme again!), and the Native Draxan campaign (which would have to be highly divergent from the regular campaign).
My overall impression is that of a good book that is well executed, but ultimately terribly limited by its own extent. A regular Dark Sun campaign will find good use of the opening chapters for near the Sea of Silt, but the rest of the book -- particularly Ur Draxa -- is mostly of little utility since nobody will go there, unless they're insane, psionically dominated to do so, made slaves, or suicidally want to alpha strike the Dragon. Only 2.5 stars out of 5 from me. It would be much higher if the utility was higher. Don't bother buying it unless you're really really intending to use this area of Athas.