Best Django 'CMS' component for integration into existing site
I have worked with all three (and more) and they are all built for different use cases IMHO. I would agree that these are the top-teir choices.
The grid comparison at djangopluggables.com certainly can make evaluating each of these easier.
django-cms is the most full-featured and is something you could actually hand over to clients without being irresponsible. Even though it has features for integrating other apps, it doesn't have the extensibility/integration of FeinCMS or the simplicity of django-page-cms. That being said, I think the consensus is that this is the best Open Source CMS for Django. However, it's docs are a little lacking. update: I have been told that integrating apps into DjangoCMS 2.1 has been improved.
FeinCMS - Is a great set of tools for combining and building CMS functionality into your own apps. It's not "out of the box" at all, which means that you can integrate it however you want. It doesn't want to take over your urls.py or control how you route pages. It's probably a prototype for the next-generation of truly pluggable apps in Django. - We are moving from django-page-cms to FeinCMS because our primary models is high volume eCommerce and I have custom content-types I want to integrate that aren't blogs or flash. Good documentation and support as well.
Django-page-cms - Is great if you want to just have some "About Us" pages around your principle application. Its menu system is not truly hierarchical and building your page presentation is up to you. But it's very simple, unobtrusive, and very easy to slap into your app and get a navigation going that clients can manage, or even for yourself. It has no docs that I know of, but you won't really need any. Read the code and you will get it all in 30 minutes or less.
update
Mezzanine - Is a very well designed CMS and one that I have finally settled on for most of my client work, mostly because it has an integrated eCommerce portion. But beyond that it has very extensible page models, and a custom admin interface that a client might be willing to use. It also has the best "out of the box" experience i.e. You can have a full fledged site up with one command.
If you do not necessarily want a finished CMS with a fixed feature set, but rather tools on top of Django to build your own CMS I recommend looking into FeinCMS. It follows a toolkit philosophy instead of trying to solve everything and (too) often failing to do so.
http://github.com/matthiask/feincms/tree/master
Disclaimer: It is my brainchild, and the result of too many frustrating experiences trying to customize another CMS for the needs of my customers.
There is also this one that is quite nice as well: