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Pro Active community
Yeoman has an active community with new generators being created at a rapid pace. Because of the momentum behind the community, you can expect good support and adoption for new tools and frameworks promptly after they come out.
Pro Huge number of generators for scaffolding your project
Yeoman generators allow you to quickly set up a new project. Invoked with the scaffolding tool 'yo' they provide a boilerplate & tooling selection. There are over 1000 generators, including generators for ember, angular & backbone, to choose from, the majority of which are community maintained.
Pro Allows you to choose between different build systems
Yeoman supports both major build systems - Grunt and Gulp. These build systems will help you automate tasks such as minification & concatenation of files, running tests, deploying and live-updating your webpage among many others.
Pro Support for the package manager of your choice
Yeoman supports both Bower and npm, and is flexible in regard to tools to allow it to work with a wider range of project requirements.
Pro Free and open source
Yeoman is free, open source and licensed under BSD.
Pro Standardized workflow process
Yeoman wants webapp development to be more standardized under the "Yeoman workflow" banner. As such it encourages the use of a specific combination of tools - a scaffolding tool (yo), a build tool (grunt, gulp, etc) and a package manager (bower, npm).
Pro Generators can be composed with other generators
Yeoman's scaffolding system allows generators to rely on other generators allowing for better code reuse and standardization between generators that use a common sub-component.
Pro Works with the package manager directly
Yeoman doesn't just scaffold your project, but also helps you integrate with your package manager directly, so you can manage your entire project with it.
As a command line tool it works on OS X, Linux & Windows.
Pro Developers can create their own plugins
Developers can also create their own Yeoman generator which are practically plugins with which Yeoman works. Generators are basically Node.js modules and can be created just like any other Node module.
There is also a very detailed and useful guide on how to create a generator on the Yeoman official website.
Con Combining Yeoman and backend frameworks can bring problems
Combining Yeoman and a backend framework such as Django, Rails or Laravel can create problem because the project structure of Yeoman may not be compatible with that of the backend project. It can be tuned to work but for small projects it can be relatively time consuming.