I have used accessory a couple of times now.
Generally I like it. It is easy to use and does a good job.
I have only given 4 stars instead of 5, because I find the adjustment knob very stiff, and the range of adjustment seems quite coarse and limited. It does not seem possible to simply shut off the foam completely, so if I want that I actually stop and remove this device from the lance - but it would be nice to be able to just shut off the foam completely.
It does not feel that there is a gradual (or linear) reduction of foam from maximum to minimum.
I certainly use more product with this than if I used a bucket and sponge - but I can live with that, it is not a surprise. It is certainly MUCH better then putting a product directly into the tank of my pressure washer.
Incidentally , comparing the price of this attachment with other similar options from Karcher currently available on Amazon, this one is definitely value for money.
The engineer in me will probably drive me to dismantle this and see how the adjustment should work, and whther or not I can improve it.
I am happy with it and keep using it.