CityWorks belongs to Fantasy Flight Games's "Legends and Lairs" sourcebook series. The author, Mike Mearls, now works for Wizards of the Coast on the Dungeons & Dragons development team, and his thorough familiarity with the rules set adds robustness to the product. On the other hand, CityWorks was published in 2003, before the advent of the D&D 3.5, so the product presupposes the older D&D 3.0 rules. DMs seeking to use CityWorks "crunch" in their 3.5 campaigns may need to update some material in the book, for example, reference to 3.0 skills like Wilderness Lore that dropped out of or were renamed in 3.5. On the other hand, as described below, much of the material in the book is completely edition-proof and, indeed, system-independent, so even DMs who don't use any form of D&D or d20 rules can still benefit from the book.
Chapter 1, aimed primarily at players, presents various options of the sort you'd expect: advice about running standard character classes in an urban environment, new base and prestige classes, new feats, and new spells. As a DM, I would be comfortable with most of these options in my own urban campaign. I might want to reserve the assassin class and the kingpin prestige class for NPCs, and the pit fighter class seems to be a bit too "niche" (I would rather have players choose a fighter or barbarian and fill in the "fight club" angle through role-playing and backstory). The most innovative offering is the concept of "urbanmancy" or city-based magic (though the metaphysics of why there should be special magic in cities is left wholly unexplained). To Mearls's credit, he does not overload the chapter with silly or redundant feats or spells, limiting himself to fewer than a dozen feats and as many spells (of which all but three are urbanmancy "prestige spells" usually requiring the urbanmancy feat or the speaker of the city prestige class as prerequisites).
Mearls uses chapter 2 to survey various aspects of a city's "story": origins, history, laws, holidays, government, and so on. He helpfully provides six "city archetypes" which DMs can uses as "shortcuts" to fleshing out their cities. DMs whose PCs visit many different cities over the course of a campaign should really benefit from the archetypes. Chapter 2 deals chiefly in so-called "fluff," which works to the advantage of this older sourcebook. The book's "crunch" is all 3.0, but chapter 2 is about equally useful whether you're playing D&D 3.0, D&D 3.5, D&D 4e, Pathfinder, or even AD&D. About the only system-specific material in chapter 2 concerns monetary systems and alignment (along with a few stat blocks for city guards), and DMs can very easily make the requisite tweaks.
Mapping a city can intimidate even experienced DMs, and it often takes a long time. Mearls helps DMs out in chapter 3 by describing a simple process for laying out a city. Old-school DMs will get a sense of nostalgia from the random tables that Mearls has constructed for this chapter: block proportions, city shape, major streets, and so on. Mearls has anticipated many possible variations (cliffside cities, subterranean cities, and so on). The material in this chapter works perfectly with any fantasy RPG rules set.
Chapter 4 manages to stay mostly edition-proof as well, for here Mearls describes principles of adventure design that would work well with any fantasy RPG in an urban setting (and maybe even for some non-fantasy settings, such as superhero RPGs). Urban adventures offer PCs many more options than dungeon crawls, and Mearls does a good job of helping DMs map out the relationships between various events that might happen over the course of an adventure. Chapter 4 also offers a system for chases through a fantasy town's streets, though DMs who want a "crunchier" chase system for d20 games might do better to check out Adamant Entertainment's "Hot Pursuit" and "Hot Pursuit on Foot."
Mearls fills chapter 5, "City Encounters," with a cartload of random tables. Need a quick idea for an encounter in the market or a public park? Just pull out your d% and roll on one of Mearls's encounter tables. He even provides a quick set of tables for creating NPC names on the fly, though a product like the old Judges' Guild "Treasury of Archaic Names" or one of the several online random name generators will give you much more varied results if you have time to consult them. I found this to be the chapter I'm least likely to use on a regular basis.
All in all, CityWorks is a very nice product. Enough of the text is edition-proof and system-independent that it remains quite useful, even though the "crunch" is based on the 3e rules. I would go so far as to say that this book is better than WotC's Cityscape, and I'm glad that it's part of my library.