Hi Linus,
Yes , In ruby world also we use CMSs a lot (Like you said, LocomotiveCMS, RadiantCMS, Refinary etc...), But having said that sometimes its good to create a project from scratch as you have the full control.
I have good and bad experience in having CMSs as the base project, I had one project (which I inherited from another party to finish) which was developed for a specific client need, but unfortunately the initial team has used a CMS and it was very had to implement some of the very straight forward feature (in pure rails) as I had to compliment to CMS workflow.
But on the other hand, I had some good experience with CMS as sometimes the client want a basic website with editing pages etc.. so CMS would be your ideal friend (I personally prefer Refinary (
http://refinerycms.com/) due to simplicity and rails3 support)
And I'm not a big fan of ActiveAdmin, but I do use twitterbootstrap (
http://twitter.github.com/bootstrap/) and its rails gem (
https://github.com/seyhunak/twitter-bootstrap-rails/) to make my pages pretty, I guess its worth to have a look.
and for simple CMS kind of features you may use the 'liquid' gem (
http://liquidmarkup.org/) as well
So all in all, using a CMS or not is depending on your client requirment
hope this helps
cheers
sameera