I've used my old Bronica S-series chimney finder as a loupe as well, both for transparencies and on the groundglass. It's Nikkor glass, and the square base is nice for getting in the corners of the groundglass.

For transparencies and groundglass viewing I like the Schneider 4x. The current one is much better than the old one. You can see a whole 35mm slide with it with no distortion and completely even illumination, and less eye strain as a result.

To check sharpness for enlargement potential, you might want to have a 10x as well.