Snow Foams are effective when used correctly and when the correct product is used.
They are, essentially, pre-washes and not a substitute for proper hand washing but in the winter they can be sueful for removing some of the road salt and grime without resorting to a full wash.
Don't be misled into the cheaper 'snow foams' - check the dilution ratios and then work out how much each litre of solution is going to cost you:
I did a wee comparison between Meguairs' Hyper Wash (as included in in our foam lance kit
Special Offers | PB Foam Lance Kit) and AN Other Snow Foam: AN Other snow foam: light soil dilution ratio: 25:1 water/product (or 15:1 heavy soil)
5litres at £14.99 plus £15.94 shipping = £30.93.
At light soil dilution, 5 litres equals 125 litres of solution.
Meguiar's Hyper Wash: standard dilution is 400:1 (water/product).
3.78litres at £29.99 plus £8.95 standard next day shipping = £38.94.
3.78litres @ 400:1 = 1,512 litres of solution
Cost per litre for Hyper Wash at 400:1 (assuming shipping at £8.95) = 0.025 pence per litre (yes, 0.025)
Cost per litre for AN Other Foam at 25:1 = 0.247 pence per litre so almost ten times the cost.....
With regard to the foam you get at wash points: that's traffic film remover: heavily alkali, very good at stripping dirt but also good at stripping protection, staining trims and generally attacking both the clear coat on your paint and the lacquer on your wheels.