I can't improve on what Vox said, but I do have a different recommendation. For any home owner or even renter who has a lot of carpet, or even a lesser amount of carpet that gets dirty quite often, I recommend buying a small one-piece professional extractor.
I bought a small demonstrator from a source I know and trust. Later I added a wand for stairs, a tool for drapes and upholstery, and a 20 foot hide-a-hose. I'm really set, and it only cost about $2,000. I know that sounds like a lot, but the results are worth every penny.
The extractor has five jets and a horizontal, dual chevron brush. The added tools are an invaluable addition. I make my truck upholstery look as good as new! Machines such as Rug Doctors can't come close to the efficiency of this unit. I know this isn't for everyone, but it really works for me.
Whatever it takes.