We hired one and followed the instructions to the letter. After finishing (and yes, the carpet and rug did look clean) we started noticing a musty smell. This got progressivly worse over the next 72 hours. We ended up throwing away a £300 rug and having to get carpet replaced in two beedrooms. The underlay had got saturated and gone mouldy. The smell was awful. Rug Doctors dealt with it appallingly, they eventually took the machine away from B&Q for testing and called back saying there was nothing wrong with it. All the chasing was done by me, they didn't return calls or respond to emails. They offered no compensation and refused to allow me to get the machine independently tested.
It cost us a fortune in putting it right and I wouldn't use them again.
Edit, plenty of other similar experiences on Google, try a couple of careful searches and see what comes up.