It's probably because I'm expecting too much but every time I've used a carpet cleaner I've been disappointed.
I think it is because if you have carpets that are marked in distinct areas they will still looked marked after cleaning.
If your carpets have had even wear and have become ingrained with dirt evenly over the years then the years then a carpet cleaner will deliver good and pleasing results.
My advice would be to hire a cleaner for a day or two and see how you get on.
Many carpet cleaning companies are shysters in the way they operate. Some will offer you a great price to get into your house but then will roll out "this is bigger or more complicated" or the favourite "well we could do it at that price with our standard product but if you want the job doing well you should upgrade to our super product"
Hard work, and be prepared for slightly damp carpets afterwards - the cleaners do remove most of the water but they can only do so much.
And clean before you move furniture in if possible.
Cheers,
Nigel
An example of what I'm talking about. In my lounge we have a beige carpet which was bought new, mid-price, scotchguarded. After a couple of years it showed an ingrained dirt 'path' from the doorway, and a patch where my children would sit on the floor playing XBox.
Tried hand cleaners, then a rug doctor like machine and finally got the professionals in. Although overall the carpet got cleaner the marks are still very evident.
So yes they clean the carpets but don't expect the carpet to look new and unmarked when finished.