Dang it. Had I not just bought a (much cheaper) Hoover! I rented one of these once and it was great.
I shampooed a rental with a hoover recently it did just fine and got up a lot of dirt. I think they are fine for the occasional user. Only downside I found was having to refill it with water / soap every 10-15 minutes of use.
If you plan to shampoo more than once a year, this is probably worth the investment, otherwise the Hoovers are just fine.