Plenty of people use Illustrator every day. If you've read the introduction to my book, you know that there are all kinds of uses for Illustrator ranging from general graphic design, to fashion design, environmental graphics, and packaging.
Many architects and engineers also use Illustrator to design buildings, layout interiors, and specify mechanical specifciations for products that we use every day. However, such work requires extreme precision and such work is commonly done in dedicated CAD (Computer-Aided Design) applications and systems.
Brendan Cheves always wished that some of the precision tools found in CAD applications would exist in Illustrator and so he set out to create them. Through his company, HotDoor, Inc., Brendan created CADtools, a plugin for Illustrator. Currently in version 4, the plugin adds a plethora of tools and functions to Illustrator including the abilty to draw and work in scale, automatic dimensioning, isometric capabilities, and automatic labeling.
Above and beyond the tools, HotDoor also has several other items to support CAD-type use including a full collection of patterns to simulate environmental textures (stone, wood, brick, etc.) and collections of symbols as well, specific for elements like landscaping, electrical, etc. CADgate is also a utility that can be used to convert files between many different CAD formats.
Brendan also developed a plugin called MultiPage (currently in Version 2, although support for CS2 isn't available yet). This plugin allows you to simulate multiple pages in Illustrator, using Layers.
For more information on these plugins, check out www.hotdoor.com.