I finally managed to try this out, as the temperature today struggled above 7c, which is my cut-off point for car washing.
I bought the attachment for using a drinks bottle (I used a two litre mineral water bottle) which worked quite well, but needed to be refilled to allow me to rinse the whole car, including the wheel arches. Connecting the washer to a hose appeared to give a little more pressure, and I experimented with the adjustable nozzle, which is useful, and put some snow foam concentrate in the shampoo dispenser. This produced a somewhat watery foam, which didn't "dwell" particularly well, so I might experiment some more with this. The results made me think that it would work well with a citrus pre-wash, though.
It's powerful enough to remove some, but not all, of the grime adhering to the paint, but it's better than mains pressure.
The device works as a portable light-power jet wash, so it would be ideal for something like mountain bikes as it produces enough pressure to deal with mud that hasn't dried and caked on. At 10bar it's never going to produce as much power as a mains pressure washer, but then it's not intended to be used as one. The big attraction with the Worx kit is its portability and for that, and the fact that it's more powerful than mains water pressure, I'm quite happy with it.