Hi Jeff,
I recently bought 3 Vikan sill brushes, and changed all my poles over to them and to be truthful I must agree with Tosh that they only add weight to your pole, it might not seem much but when its at the other end of a 38' pole you know about it.
Although one thing I will take from using them is I'll always use them on first cleans cos they do get into the corners of the frames better and I'll leave the one on my 12' pole cos the added weight isnt a problem on such a small pole and because I use it for mainly ground floor it does leave the frames cleaner, and the ground floor is the most likely floor that fussy customers will inspect.
I would recommend you buy one cos they last forever and will always be of use
Hope that helps
Alistair