Recently I picked up one of these little lenses from a surplus seller on Ebay that has a batch of these for sale. It has impossible to pass up for $20 but how does it perform compared to a lens like the Canon MP-E 65 at 3.4x? Better than I expected!
Check out the full test here on Closeuphotography.com: https://www.closeuphotography.com/copal-e18c-lens/