I bought one of these filters, and I had a slightly different experience to others. First off, let me say that it does exactly what they say it does - you get 0ppm water out of it. I tested mine with a TDS meter; it did what it said. And the input TDS was around 380-400 (Surrey).
Those using an RO stage - the problem with RO stage is that to get the lowest ppm output you'll get a pretty poor flow rate, which is probably why Geoff's system is 18ppm - it'll be to do with the hole size in the RO membrane. An aquarium doesn't need a high flow rate so you can use a very fine membrane to reduce the ppm as low as possible. Maybe car rinsing doesn't need much flow either (see later).
I used my filter for around 3 months and then it ran out. I was leaving the car to dry naturally and this is where I got caught out. Even though I had a TDS meter I didn't expect the filter performance to suddenly drop off the way it did. The result was a heavily water spotted car one day that I had to do paint correction on to sort out. Luckily I had the gear to do it, so it was only my time.
I checked how much water I was using and found that a rinse of my XKR using a hose would cost me 25 litres of water. That was trying to use it sparingly too. At 400ppm the 7 litre filter will give you just under 400 litres of water - so only 16 washes in my case. Admittedly I had also used the filter for washing at first (snow foam) which is partly why it ran out so quickly, but I soon stopped that when I realised how much water that used. This would still mean though that with my usage it might only last 6 months maximum and that's being generous. I could have tried to get the water usage down but frankly I couldn't be arsed with it.
I never refilled it as I felt it would just be too expensive and a right pain in the water butt to refill it every 3-6 months.
Other's mileage will very - it depends how anal you want to be about using the minimum amount of water, but I'd say if you want to go out and do a quick wash with little concern for what you're doing then this sort of filter isn't for you. I knew all the stuff about using it sparingly (and tried) but I still failed to get good lifetime out of it.
Instead I went with a whole house water softener. This I felt was much more beneficial to me. In terms of water spotting, it won't dry clear like the ion exchanged water, but the difference between that and hard water is that the minerals which leave limescale have been exchanged for something much easier to remove. If your car does happen to dry all it takes is a damp cloth to remove any spots, and I've never even had that problem anyway. The end result is exactly the same as with the race glaze filter, and it's a lot less hassle plus the rest of your house benefits too. And on balance I prefer to dry the car anyway.
This is just another perspective - there's nothing to fault the filter with other than the maintenance aspect which I didn't like.
In both cases, if you live in a hard water area your car will look visibly cleaner after a wash and dry than it does with hard water. It's really obvious the first time you do it how much deeper the colour is. I found this particularly true with a black car. It still amazes me every time I dry the car.
Personally I wouldn't say that the race glaze filter is better than a water softener, but realise some people might prefer it. For me, it's actually the other way round.