I bought Gardiner's sill brush, more for cleaning fascias and getting behind gutters etc, than anything. Tried it on the glass and really can't get on with it. I found it did a good job on the sills. However, on the glass it was a different story. First, as has been pointed out, due to the way the bristles splay the stock keeps hitting the frame. second I use a swivel and this seems to make the stock hitting the frame problem even worse. Oddly too, I don't find it anywhere near as effective on the glass as a good standard dual trim brush, like the Ultimate, Supalite or the X-Line, mono or flocked. I currently find myself switching between the Gardiner's Ultimate Flocked and the X-Line Evo-Lite depending on the job
We all have our preferences. Just have to try them and see what suits us and the kind of work we each do.