Hi Martin,
I have window mates and use them occasionally, did today as a matter of fact
They can be a bit tricky to use but with practice I have got pretty good results, put it this way they will pay for themselves if you use them a handful of times.
Quality of the rubbers is not that great and unfortunately they dont seem to take ordinary ones, its quite difficult to clean into the corner of windows with but i explain that to my customers, they still seem happy, its that or dirty windows afterall, so I guess you have to weigh it up really.
Tosh maybe right in some respects, I think they are aimed more at the domestic market but with a bit of patience theyre ok.
Simon
p.s. I got the stronger ones for double glazing though i use them on single glazing too, just make sure you use plenty of water/liquid or some rinse aid/ softner in your solution you'll need plenty of slip