Where to put plain object classes?

What's your point? You don't want to use the CI_Model class at all?

My point is that it doesn't seem worthwhile to use it so why should I. I'd even go further to say that it might be detrimental to use it because, as mwhitney confirms, it would mingle the properties of my model class with those of the CI object due to the magic __get method.

Why would it cough up CI properties?

See mwhitney's post. Also, PHP docs on the __get magic method might be helpful.