ExpressionEngine is used by myself and by a number of my clients to make managing the company website dead simple.
- Accommodates any custom design
- Ease of use for non-technical clients
- Scalability and flexibility for future growth
- Third party modules aren't always up to date with current version.
- Updating process is a bit cumbersome.
- Clients actually update websites built with EE because it's simple to use.
- Conversions are more a function of UI / design but I will say last week I had a client inform me that they had a 34% increase on a contest they run after a redesign done in EE.
In my opinion ExpressionEngine is a far better system than these primarily because you aren't inheriting pre-built front end code (all of these systems work on this model) and clients have found EE easier to use than even custom configurations of these three.