I've never, ever been impressed by snow foam.
I've bought the lance from the group buy on DW - HD lance (something like that) and bought a gallon of SSP and Magic Foam (?) - can't remember the exact ones.
Long story short, it doesn't do much/anything. I've tried it on absolutely filthy cars to slightly dirty cars and have always had to resort to a hand-clean.
I bought it so that I could do a weekly touch-free wash, but it certainly won't ever replace a hand-wash, it does nothing!
... But as above, I still use it - it amuses me to fill the driveway with 2" thick foam.