Ian,
We have used one for years. It is an indispensable piece of kit.
It is basically, a really heavy duty industrial vacuum cleaner that fetches out the crap from deep down in the carpet.
It only has wheels at the back. All of the weight is supported by the brush (think a HOST type brush). This is powered by a 3/4 horsepower motor that is geared down. All the weight is taken on the brush which really does get down to the carpet backing.
It drives forward at a fast walking pace but the real results come when you pull it backwards (against the drive). This (to paraphrase the playtex ad) lifts and separates the pile. It really does help the cleaning process.
Even on low profile stuff (as in this video
http://www.youtube.com/watch?v=0ZX-vhqFj2o ) it still gets a load out and breaks up the surface of compacted dirt.
On some jobs (say a domestic through lounge) you can find yourself in a situation where the carpet looks completely clean after just the pre-vac.
When you finish the vacuuming, more often or not, you have a bag full of dust that is almost like talcum powder. This I presume, is the impacted stuff from deep down.
You can probably tell I'm a fan.
Garry