I had a customer the other day who had used a Karcher window vac he'd borrowed. From the outside I could see a few marks which to be honest didn't look too bad if you're not too fussy. However, when I called for my money I could see through to the kitchen window from the front and was shocked (The sun was shining through) they looked terrible. I was fully prepared to go back round and do them again as I thought I must have missed the window until he said, 'Oh no, that's not you, that's the Karcher I borrowed.'
What most people don't realise because of clever advertising, is that you can do the same, or as in my customers case an even a better job on most windows with a little water from an £1 atomizer bottle and a proper microfibre (£6) which will last them years. Microfibes don't have the novelty factor though. They look too much like real work