I got one of these a few weeks ago, and I think it's badly designed...
When it's giving out normal tap water it's too powerful. Firstly I wouldn't want to use a strong get to remove dirt from my car, and secondly it splashes everywhere. If I used it correctly I would splash our houses windows, and my neighbours car. Also if you're washing the car in sections (as the instructions tell you) then you'll splash the sections that have already been washed/dried. The device it also too big to get under the wheel arches etc., so a normal attachment is required for a thorough wash.
The soapy water gives out far too much, and you have no control over the flow, just an on/off switch. A bucket is much better! The soap also leaves a horrible oily film over everything, which is a nightmare when driving with the wipers on. After a week with VERY heavy rain, and another wash, most of the oil on the windscreen has gone, but it is still on the rest of the car. I'm not using the soap again.
The de-ionised water sprays at a very slow rate though. So it takes a very long time going over with the auto dry. However, with the auto dry I can be sure that the bodywork won't be scratched. The roof came out very well, but other part of the car had water marks left behind. This may be due to the soap, but I'll have to see as time goes on. The de-ionised water costs around 50p/wash too, which I wouldn't normally pay for a normal wash.
You can get a full refund during the first year, but I haven't got my receipt, so I suspect that I won't be able to get my money back :-(