Shadows of a Dying World is loyal to its source material. Excerpts from the novels are used as part of the monster descriptions. I especially like the fact that some of the monsters can be used as characters (using the Racial Ability Scores Modifiers - a hallmark of 3.5e). I'd love to play an 8ft tall four armed green-skinned Thark.
D&D plate-mail doesn't really have a place here, in fact actual clothing is fairly rare in ERB's Martian books. So Appendix III in Shadows' provides us with Class Defence Bonuses - which give players just enough of a level-linked boost to AC so that even when they're standing still characters may still survive the slings, arrows and radium guns of outrageous Barsoom. Loin-clothed barbarian-types are welcome here.
With Feats descriptions and Random Encounter Tables, this product comes over as a succinct, settings based, Monster Manual with extras. It's all generic enough to make the Mars milieux your own. Skirmisher Publishing have also included a document built from select tables and text in the SRD to assist with creature building and modifying what's already provided in the text.
Because this is an OGL product, in theory all you would need is the d20 Source Reference Document and this product to play campaigns on Mars (but if I remember right, the SRD doesn't include character generation so you'll probably enjoy this more if you actually own the rules for Pathfinder, core DnD3/3.5 rule books or d20 fantasy-equivalent).
The many interior illustrations are in a variety of styles, black and white and printer friendly.
For many DMs who are fans of this genre, who use a d20 system, this document will be perfect starting point for encounters with monsters based on adventures from one of the grand-daddies of fantasy.
I'm really grateful that I've had a chance to see this product. It's certainly worth the $6. It is definitely worthwhile considering if d20/PF/DnD/3-3.5 is your preferred system and you're dying to try out some truly classic, but brutal, sci-fantasy. :)