I have the SkiVisions tool & it has worked pretty well for me the last couple seasons. So, some info on my experiences from using it...
For flattening skis, I only use it for skis that are just slightly concave/convex. Extreme cases can just take too much time for me with the tool (like days), so I get those skis ground at the shop.
I use the tool most often for structuring. I have both the medium & coarse stones. Probably use the medium the most. I'll sometimes use the coarse stone to prep for spring skiing late in the season.
Where the tool also really works well for me is the final step of base repair. I use the metal cutting bar to flatten the repaired area & then use one of the stones to impart structure on the repaired area (if needed). Gives me consistently good results. But, full disclosure here: I can do the same thing (minus the structuring part) armed with just a metal scraper with a good burr on it.
I have tried using the stones alone for tackling some challenging spots on the base or to fine tune the structure. Simply put: it can be tricky. You really have to hold the stone carefully to get the pressure & angle consistent - but the stones are pretty narrow and thus can be hard to control by hand. The tool (which is really a specialized holder for their stones & cutter) really is needed if you want consistency & accuracy. You do have to clean the stone (with a brass brush) every couple of passes, otherwise the stone will get clogged & not cut well. I guess the main rules in using the tool: use the hand position shown in their videos, use moderate pressure and clean the stone every couple of passes.
The stones are about 1/2"x1/2" diameter. Two sides (opposite of one another) have a special abrasive grit deposited on the surface. The other two sides are plain stone. You "freshen" the sharpness of the cutting edges by sanding the plain (non-abrasive) sides on 220 grit emery paper (paper is on a very flat plate of metal or glass). It only takes a few passes on the emery to get the sharp edge back.
For most efficient use of the tool: make sure your bases are really really clean & free of wax. Otherwise the stone will clog with wax the first pass - and that can be a pain to clean. Experiment with the orientation of the stone in the tool to figure out which orientation cuts best. In my experience, stone orientation for best cutting can differ from stone to stone.
I imagine the stones are probably brittle as they are pretty narrow and, well, they're stones. Haven't broken one yet.
