I bought this machine as it seemed super powerful and I'm a student with cream carpets in my flat and alot of friends who "forget" to take off their shoes and clean up spills.
I haven't been disappointed with regards to the carpet cleaning function. O.K. so you need to set aside an afternoon for doing this, and my gosh is it a full workout when both compartments for water are nearly full but it really does bring the carpets up like new. One or two really deep rooted stains have faded rather than gone entirely, although I'm sure if i ketp at it they'd go eventually. My only major disappointment is with an unknown stain (looks like hair dye) that was left for a week (while I was away) before being shampooed - but seeing as washing up liquid, biological washing powder, 10 applications of Vanish and 5 applications of another stain removing product have not worked I feel maybe this one is my friend's fault for leaving the stain and not the shampooer's fault for not being able to get it out. A 5 minute application of vanish oxy action (which didn't seem to do much to the stain to be fair - it definitely wasn't vanishing before my eyes!) coimbined with the VAX upholstery nozzle alone got a 3 day old red wine stain completely out - I think that is adequate testament to how good this machine can be.
The upholstery attachment is disappointing, I have been left considering a steam cleaning unit for my cream sofas as the upholstery nozzle doesn't have the same deep cleaning system as the carpet, and I've been left with some greasy marks even after two (albeit breif) shampoos.
Having to refill the warm water tank frequently is a bit of a nuisance - but that's where I get confused because the waste tank is much larger capacity, so that doesn't need emptying as often as you need to refill the clean water - but I'm sure that's probably because that's a less pleasant and more tiresome job than filling the clean water, and the manufacturers had to compromise on amount of water somewhere due to weight and manoueverability.
The carpets do take at least a full day with the windows open to dry out if you've rinsed off properly - sofas etc. slightly, but not much quicker. It's worthwhile making sure you do rinse properly though - otherwise you get a sticky residue left from the soap which attracts dirt all the more and you just have to shampoo again sooner.
Also consider if you have scotchguarded carpets or upholstery shampooing will remove the scotchguard. I believe you can get spray on cans online, but they are very expensive and you need lots. Once you've removed the scotchguarding you'll need to clean more frequently as the protection against stains marks and spills has been washed away. My sofas and carpets are a few years old, well used and will need replacing in a year or two so I was happy to do this.