I picked one of these up from Robert Dyas the other day, on account of the fact it was only £9.99.
I used it on the Zed, Volvo and Lotus yesterday as a test. I'm not sure if many / any of you have got these already, but for those who can't be doing with the whole multi bucket (I use 4 normally) effort, it is a clever bit of kit. It has the wash solution built in for coating the car before mitt (NO SPONGE!) useage.
I have got an inline filter on my hose ready for use, but I thought I'd try this thing first, and I have to say the spotless dry does genuinely work. In a nutshell, it's de-ionised water (therefore almost pure) so when it dries it leaves no residue or spots. It took a lot of will power to not dry any of them, but even after a night of heavy dew, all the cars are still spot free. I even used it on the wheels.
In short, I'd recommend one (at £9.99 anyway!). In Robert Dyas, the refills for the thing are only £4.99 for 10 wash cartridges. It's especially useful for doing multiple cars as you don't have to worry about drying.
On the flip side, it doesn't help if you want to polish / wax.
I'm yet to use it when clay barring, which I think could be the real gain.