I used my Vikan sill brush on a twelve foot extender the other day - it's like cleaning in slow motion it's that heavy. Jeff have you tried a Bentley yet? It's not quite got the scrubbing power of a Gaz brush but they are soo comfy to use. I think Alex's new version will be the one - when it comes out.
I have got a bentley Dave, (brush that is

) I used it on a couple of Jobs, they are so light and easy to use, but I found it gets contaminated at the tips to easy,
I was having to stop and clean it far to much, it may have been the couple of Jobs I did, but I went straight back to my Gazz brushes, i don't use anything else now.
I know a lot of guy's use the bentley and are more than happy with them, I may just give it another try one day when I'm not rushing around like a demented cat.