I was disappointed to find the secondhand Bronica lenses I bought were single element, not the supposedly better cemented doublet. However, they only cost £15 and are multicoated, unlike the cheap "Regent" brand type I had before. I have not detected any obvious difference, but I really haven't looked too hard.
The Hoya lenses are double coated. Multi-coating may have the theoretical advantage of less flare, but if you shade the lens properly (which you ought, as these lenses are just hanging out there, without any form of shading at all) I suspect that any differences will be minor.
I have read that aperture is important with supplementary lenses. Every lens will have a best aperture. Shooting away from that aperture will decrease the performance of the lens. Apparently, putting a supplementary lens on the front will make worse the decrease in performance; single element more so than cemented doublet. Shoot at the best aperture, and these lenses can perform very well (especially the cemented doublets). Of course, for close-up work, you may have to shoot some way off the best aperture in order to get the depth of field you want. If this worries you, then its time for tubes etc.
BTW, I remember SpeedGraphic (
www.speedgraphic.co.uk) selling the Nikon 62mm close up lens for £60+. As this would be the cemented doublet referred to by a previous poster, it makes the Nikon lens much better value for money. However, I couldn't find it in the current online catalogue.