I have to both agree and disagree with those who say the Kirby shampoo system does not work well. It does not work well as an effective carpet cleaning method when used BY ITSELF! However, if it is used as a method of scrubbing the spots, stains, and traffic areas as a prelude to using an extractor, it is extremely effective. Why? because the bristles do indeed deposit much of the dirt they loosen into the tray. They rotate at a high speed and scrub harder than the extractor brushes while laying down much less water. You can actually see the black grime running into the tray on the Kirby.
The best method I have found is to use ONE capful of Kirby shampoo and 1.5 capfuls of Hoover, Bissell, or other extractor detergent in the tank. The mixture is wetter and less foamy, but more effective at scrubbing those really dirty, stained areas. Then when you extract you can use COLD water and really rinse the carpet clean. I have had phenomenal results with this 'dual' method. I do vacuum thoroughly and put down a good-prespotter, even hand scrubbing if I think some areas need it, but after Kirby scrubbing and rinse extracting,(and fluffing with the shampooer brush, you need the carpet shield to use the brush without the tray) the carpet looks great. Try it and you will be amazed.