I have one. It works well, but I can't figure how to apply the soap. My regular pressure washer wand has a low pressure setting that siphons soap out of a tank. On the high pressure setting, no soap, but pressure. The undercarriage attachment only has a high pressure setting.
To clean my vehicles undercarriage, I sprayed soap on my undercarriage from a spray bottle, let it soak, and then washed off. It works, but not as convenient as I hoped.