Like Kenny, I have the prism finder with magnifier attachment although frankly get good focus without the magnifier because most of my shots arevstopped down to maximize DOF. The magnifer is more useful when you,re wide open so there is a need for critical focusing.
The main issue is that the prism finder allows eye level view and the scene is not reversed like the waist level and I assume the chimney lens. I just don,t care for reversed images when composing the image. I,ll only use it on very low shots where the prism cannot be viewed.