Test how long the knife stays sharp, with and without stropping. If it will be dull after a few trivial board impacts, what seems to be sharp was actually just a burr - bad sharpener. If it quickly loses sharpness but can be stropped back again and again, there might be a reasonably stable wire edge, or the sharpener (and the angle it sharpens at) is just ill chosen for that particular blade - lackluster sharpener. If the edge seems stable - above average sharpener.
The problem with almost all such devices is that, even if they sharpen correctly, they still sharpen only a small area at the edge, there being a quite defined line where the sharpened area ends. While that looks good and crisp, it is a disaster in the making when such sharpeners are regularly used on a blade (worst if the edge does not last and the sharpener is utilized more often). Blades are usually either gradually getting thicker towards the spine, or there is a distinct area of a cm or two that steeply thins towards the edge. The border between the sharpened area and the remaining blade will be on a thicker section of the blade after each sharpener use. If the sharpener forces a v shape so firmly that this border will always be ground equally thin, you end up with a geometry similar to a hollow grind instead. Both are bad - you get a knife that, while being good at penetrating into ingredients at the edge, will bind in ingredients, split or plough instead of cut, and take unsafe levels of force to use. Experienced whetstone users often grind off a bit of the metal above the edge line in a steep angle each time they sharpen - even if this looks less "factory new" or might even scratch the blade.
Even with good whetstones, repairing a knife where that transition line has become far too thick easily turns into a difficult job for an hour or more.
Rule of thumb, keep non-wheeled or metal-wheel pull throughs away from knives worth more than $20, and wheeled (ceramic wheels) ones from anything above $50 (MAYBE excepting matched sharpeners that are made for and sold with a knife series).