These days, most of the help guides available for Mac apps have the CHM extension (Microsoft Compiled HTML Help). Unfortunately, Mac OS X doesn't provide you with a native app to enable you to view this type of files. CHM View is a simple yet helpful program that offers you a solution for the aforementioned issue and lets you quickly open and read the contents of CHM documents.
The app features a sidebar on the left area of the screen which comes in handy when navigating through the pages of your CHM file. You can zoom in the text content for a clearer view, increase text size, and add bookmarks when it's necessary.
Furthermore, this program also provides you with an integrated search utility that helps you find text content within the loaded CHM file.
However, you can find cheaper or even freeware solutions for viewing CHM files (some of them can be found on the Mac App Store).
Briefly, I'd say it's worth acquiring this utility as long as you don't mind spending cash on a program like CHM View to help you read CHM files. Otherwise, you can easily find apps that offer you the same amount of features and don't cost a dime.