Advances in high-index / low-dispersion optical glasses, new barrel materials, and exotic anti-reflection coatings, along with the latest in computer-aided-design / computer-aided-manufacturing), are combined to produce lenses of unprecedented quality. Diamond-cut from crystal-clear, water-white optical glass, Opteka lenses are ground and polished to a precise precision.
All that for $59.99? Call me cynical if you like but I can't see it somehow.
I've never done slide to PC transfers but a colleague uses a slide film scanner which produces very large high definition images. He's very pleased with the results but I suspect it's not a cheap one knowing him. I believe they start at around $150 but you probably "get what you pay for" in terms of quality.