The best brushes on the market are the xtremes, the best all rounder is the xtreme medium mixed ( black and white )
I very rarely use anything else for cleaning windows, I just love the lightness and control you get once you get used to them.
I have been using my latest one for just over a year and would say it has about another 3 or 4 months of life left in it
which is value for money in my books, but even if I had to change it every 6 months it would still be worth it to me.