I like Hoover Steamvacs. Don't have much experience with the V2 series. I have 2 Hoover Widepath machines of the original series, unfortunately they are discontinued now. The Hoover's clean and scrub well. I'd say they are good for the annual carpet shampooing for various stains and allergen removal.
However, if we're talking about severely stained carpets or something extroadinarily dirty you probably want something more heavy duty with highly rated PSI sprayer and more powerful extraction to ensure its a true deep down thorough clean. You might try a RinsenVac. Perhaps a local vac shop has one for rent? I've heard that Thermax vacuums do a great job and there is another thread where a user had terrific success with HOST.
I cannot recommend Rug Doctor. I rented one once at Lowe's and it worked terribly for me. I only did one room with it insteaded the intended 3 floor townhouse. That's when I found out that my local vac shop had Rinsenvac extractors for rental. I did the entire 3 floor town house with the RinsenVac straight suction extractor head, not powered brush including redoing the room I did with the RugDoctor. I was MUCH happier with results, and ease of use with machine.
Some stores like Lowe's now have Bissell Big Green Clean Machine upright style extractors for sale. I've heard some people say they're good. They look cool and my previous experience with a Bissell shampooer was that it cleaned well, the machine wasn't durable.
Whatever route you decide, I definitely recommend Hot Water Extraction. Don't fall for the round bonnet on a floor polisher routine from a Carpet Dry Cleaning company. You want something that sucks sprays in a Hot soapy mix, possibly agitating around a little and sucking out all that junk. It's amazing what carpet extractors suck out.
Best of Luck, Happy Hoovering. I think Hoover has some commercial grade Heavy Duty Carpet extractors.
