I have been doing 3-4 cars per week with the Little Green Machine. Not sure what "maintaining" ASAP2stacks was talking about but here is the method that I use in all of my customers cars.
1. Vac
2. Stiff brush carpet and vac again
3. Apply Carpet shampoo of your choice via pump sprayer use stiff brush to work into carpet
4. Use Little Green machine with only Water to extract and rinse.
This has worked amazing for me and I have had my lil Green machine for over 2 years with no issues. I like the clean head so you can see when it's pulling up dirt and you can use it until the water runs clear.
I see no reason to spend 7x what I paid to get a extractor and I did 3 cars with it this weekend outside in 95 degree heat and it didn't overheat or anything. They are well built and stout machines. The only thing I hate is I have to fill it with water 2-3 times per car.
As long as you are not an idiot and make sure you get all the dirt out, it looks professional and my customers have no clue that I don't use a $1,500 extractor.