Most of my clients are not new web apps but websites. For this, it is much more cost effective to use Concrete5 or Expression Engine than to use Yii to roll my own CMS for the project. I love using Yii, however, and was wondering if there is a good full blown CMS in the works by anyone out there. I don’t even mind paying for things if they’re good!
IMHO, I think it is not good way to write a good cms with framework. If one writes cms on top of framework it is like building something on top of another thing(for me it is strange). It should be on the ground not on the top of another big stuff.
That’s basically the difference. 
Personally, though, I often find myself doing a lot of work to get existing CMSs to do what I want them to do, and often it turns out to be faster to just do it with Yii, making use of the wealth of existing extensions.
A lot of code can be reused between websites of the same type, just make sure you design and write your code with reusability in mind:
use modules (news/articles module, product catalog module, event calendar module, etc.)
use theming to customize the HTML part of your modules on each site
factor out every bit of reausable code into components (behaviors, widgets, application components)
It can reduce development time drastically when you have to create many websites with similar structure and content.
I agree it can be cost effective, if you don’t want many custom modules, otherwise it can be more expensive, and harder.
If you develop CMS from scratch(based on Yii for example), you have more control, cleaner code, but it can take you more time for initial release.
So, it depends on project specification and client request. Both approach have their pros and cons.
I am on the same boat here. I have looked at options like Wordpress and Modx over building from scratch with yii. I think the pros and cons need to be justified by the cost and size of the project. In my opinion most the features my client needs are found in many CMS’s out there but I can see the potential for future customised features that will be hard to integrate into an existing CMS. I am leaning towards building from scratch with yii so that I can reuse and easily extend the application.