I have a CP but it is pretty much garbage. Initial shots with the new kit showed it to be of little effect. I already added a quality CP to the wish list but seeing that I do not have my "wish" glass, I am hesitant to pull the trigger. I suppose I could get the 77mm diameter and use step down rings to fit whatever lenses I have or end up with. I like the idea of keeping one on all of the lenses, but that is a LOT of beer opportunity cost...
This is getting expensive fast and the worst part is that I am fighting the urge to break out all of my old film equipment and get the darkroom setup (its been 20+ years).
Doug, I would tend to agree with your statement regarding depth, as it is something I often struggle with when composing but never really realized until recently. Depth highlighted or leveraged is what actually makes many photographs work, irrespective of the processing or other aspects of the composition. Understanding DOF appears to only be a start, as the rest is imagination and understanding how the DOF will affect the actual composition. I suppose the camera is no different than a paintbrush or any other tool, it takes talent, imagination and practice to master.