I've been using these brushes exclusively on my cajón for a couple of months now, and I'm very happy with them.
The flexibility of the grip allows your playing a lot of nuance, because you can incorporate finger and wrist movements efortlessly into your playing.
The hardness of the brush can be adjusted, and I find myself setting the plastic rings to 1/3 most of the time, but of course your mileage will vary.
A final nice touch of these brushes is that they will not hurt the sensitive surface of the cajón -- however, mine did stain the surface a bit (the brushes are black, after all) but that was fixed easily with a damp cloth.
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