i sold my spyderco sharpmaker last year to someone who lacked flat stone sharpening skills, for half price.. after i'd used it maybe a dozen times in 15 years. .
it's an extremely limited tool. you can neither apply enough pressure nor does it touch enough of the blade at once to maintain an identical angle down the length of a long blade, albeit it's FAST for what it does. if you're impatient and don't know any better it's tolerable, most especially for short blades with not very hard steel.
works fine on cheap stainless folders, compared to nothing. add the diamond rods and it'll work on expensive stainless folders.
covers a very small fraction of what is necessary to sharpen knives. suitable for fast pocketknife touch ups, if you're careful.. and not a whole lot else.
whereas stones are not suitable without an investment in skill, WITH that investment they are vastly superior ..
i own belgian, japanese, arkansas, finnish, german, and turkish sharpening stones. the belgian is 'best' (bang buck) .. the finnish is fastest on thin carbon steel (kitchen rock) the japanese is best if you enjoy sharpening, excellent results but slower than belgian.. and more expensive.. the arkansas rocks are in great part worthless, suffer from inclusions and variable grit quality, which is why they tell you to oil em.
put it in your bible, if you have to oil it, it was the wrong rock. don't oil rocks unless you're sharpening machine tools that need oiling anyway.. not for knives and CERTAINLY not for kitchen knives. gack.
rocks is the answer that worked the LAST 5000 years, you think they suddenly quit working?
give spyderco credit, their gadget actually HAS a use, i've seen some real cute doodlebug knife sharpeners that were completely worthless junk, and spyderco avoids that. . but if you want to sharpen knives, buy good stones. an orchestra's worth. with care, you might manage it with 6 good ones, or two, maybe three.. great ones. they won't be cheap. they'll likely outlive you, so decide which kid or grandkid will inherit them OUT LOUD.