It works really well. The soft smooth shape allows enough control but doesn't cause sharp indentations that ruin the fabric look of a cloak. The silicon doesn't stick to greenstuff which makes sculpting a lot easier. you can't do fine detail but for general shaping and smooth curves it seems perfect. You can see the results below in the join around the knees and the general shape of the feet (the toe detail was put in using a knife).
Fine detail can't be done but I used a knife or my GW sculpting tool for that.
It can follow the structure underneath as it pushes into the green stuff (in this case more of a positive that a negative as that meant the cloak flowed better.
A valuable addition to my sculpting toolbox and I'm going to try and get a harder chisel edged one for detail work as I love the fact the silicone doesn't stick to anything. I got mine from Hobbycraft in the UK but most art shops seem to stock them (as paintbrushes). The cost varies depending on size but they cost about the same as a good paintbrush and from what I have heard they don't wear out so should last a long while...
Does anybody else have experience of these or any other sculpting tools that they find useful? As always please leave comments or questions below.