As an SEO practitioner, I can gladly recommend C5 as a CMS. While much of SEO is in the "what" you publish, the platform impacts the "how" and can be a valuable component.

No platform or automated system can assure you of ranking well or great floods of traffic - but C5 lets you access the areas of your site that you need to use to implement good SEO.

In short, C5 does what a good CMS should do. It gets out of the way and let's you work unencumbered. You won't likely find yourself saying "I can't do ... because of the CMS."

How much do we like C5? Not only are we in the process of switching close to 80 client sites to C5, we're also changing our own site! :)

The one MAJOR change you have to make is the duplicate urls.
Example
http://www.concrete5.org/community/forums/chat/seo-and-ease-of-rank...
http://www.concrete5.org/community/forums/chat/seo-and-ease-of-rank...
The trailing slash. I submitted a bug report. I'm not sure why this isn't being implemented in a patch or update.

We have our own internal repository of C5. I implemented the patch and saw a huge difference.

I'll be happy to share the code, it just inserts in the .htaccess file. Otherwise their are some other pages with info on this.

Besides that, everything is fine:)

I'm sure if that's submitted in Github Andrew will happily include it
in the next version if we haven't already.

best wishes

Franz Maruna
CEO - concrete5.org
http://about.me/frz