Rob,
Some windows have a coating that repells the water so that it can be difficult to get a perfect finnish, but otherwise in the hands of someone who knows what they are doing by folowing a simple process and maintaining the filter system perfect results are the norm. I can't fail to be impressed every time I use my Reach & Wash system, the glass is just perfect frame to frame and the windows do actually stay cleaner for longer. Its fantastic on leaded windows, in fact I've done some church windows and historic buildings with great results, good for georgian too so long as the window frames are in good nick. If the paint is all flaking off and the putty is falling out it can take a few cleans before you rinse all of the crap away. The sort of customers you want are the ones who are prepared to pay for a good service so their windows are usually in good condition anyway.
Tom