[UniPol]

Well, I've heard that the 99% reflectivity dielectric coated Bintel (ah la GSO et al) Quartz diagonal is up there with the best, who knows, but I have no complaints with the one I use on my Polarex/Unitron refractors. I use the Tak on my Tak's, the Meade on my Meade's, the TV on my TV's, WO on my WO's, SW on my SW's, Celestron on my Celestron's, pretty silly I know but that's just the way it is. Even if one tries to compare various brands/quality/cost the results would be so subjective for obvious reasons.

If you really want to see what effect/affect a diagonal makes to the optical train, just remove it and look straight through the eyepiece just like Gallileo (and the majority of Japanese amateur astronomers ?).