ExpressionEngine is a commercial product built on an Open Source foundation, giving you the best of both worlds. As a commercial product, ExpressionEngine is supported by a team of committed developers and technical support specialists. With its Open Source foundation, ExpressionEngine's codebase is 100% open, transparent, and extensible. For a decade we've operated on this effective business model. Our customer satisfaction and growth is a testament to its success.

Pros of ExpressionEngine :

1. It Provides Excellent security track record and offers other security features like IP Banning, Email banning and Throttling.
2. It is easily extensible, if one has knowledge of PHP. Also, one can easily extend ExpressionEngine through Extensions, Plugins and Modules.
3. ExpressionEngine allows you to cache individual templates, queries and optimize your site down to to the EE tag level.
4. Because it’s commercial software, real resources are dedicated to writing quality documentation for ExpressionEngine.
5. It provides Customizable Control Panel in which one can add tabs, remove tabs, add links and write an extension that completely changes functionality. You can also theme it the way you’d like. Tailor the control panel to the project or client. The best part is that in the coming new version of EE, all the customization is supposed to be even easier.
6. In the new version of EE, all add-ons will be written using the PHP framework CodeIgniter. For anyone that has written add-ons using the current API, using CodeIgniter will be a welcome change.
7. Custom form field.
8. It is easy-to-deploy website in EE.
9. Has active forum with a small technical support staff employed by Ellis Lab and a wikipedia that contains useful ExpressionEngine tips and tricks.

Cons of ExpressionEngine:

1. Image upload / organization could definitely have been solved more elegantly.
2. Automatic thumbnails or scaling of images only available using a third party (non-certified) plugin.
3. Some default settings are for bloggers, not professional sites. Most professional users may want to turn off these functions.
4. There got to be an easier way to loose the index.php in the URLs