As long as the stone is dry, pencil eraser works very well. (as far as I can tell the trend cleaning block that comes with their stones is a common or garden pencil eraser repurposed)
There's more to sharpening than shaving hairs off your arms mignal. Some of us like the backs of our plane blades to mate accurately with the cap iron so shavings don't clog. Many like the backs of their chisels to reference the surface when pairing. It's not going to be the end of the world if you're chopping mortices or shaving your arms perhaps, but plenty of hand tool woodworkers will consider a decent standard of flatness to be basic functionality.
To be sure, I think the review is very fair. It sharpens, it's not flat. But my take therefore is that many hand tool woodworkers would do well to be a bit more fussy with diamond stones. Flatness is useful. Indeed, one of the great advantages of diamond stones is that they provide a stable surface for lapping that doesn't go out of true. Assuming it's true to start with.
I think woodworkers who's budget might not extend to flatter diamond stones would be better saving up some more or using natural stones which you can flatten yourself. And I'd hate to think that someone on a tight budget would waste cash on something that's not as useful for them as it could be because it's been recommended enthusiastically here. You don't need to spend big bucks to work wood, but that doesn't mean you don't need to be a bit savvy.
That said, you'll get an edge that cuts. But you'd probably get that from an even cheaper stone too.
