This is my current opinion and I`ve never seen one in the flesh.
They are small,the edges of the stock are sharp corners and could potentially cause damage, a forum member Alex, says they are brilliant but also touts all things tecbuk including a book they sell which he purports to own and is full of great info though he would rather not share this,there are many variations,nylon,polyester etc and no detailed info as to why they differ,the site is not user friendly,they are a massive price,though recently seem to have got cheaper but then this seems to just be one type of brush.