I'm a hobbyist and I picked up the Gilmour Foamaster II. I like it, I don't use it every time I wash, I find the process of actually covering the entire car with adequate suds and washing the car via 2BM lets some of the soap either sheet and/or dry completely off/on the paint unless I have a really nice cloudy day to do it on, even then, I have to rush.
That being said, it's fantastic with a little bit of shampoo and a douse of APC to tackle wheel wells/wheels, cowls, exterior nooks & crannies to hit with a brush, gas caps, etc. I find myself using it only on thorough cleanings but I don't regret the purchase.
Maybe one of these days I'll finally find a pressure washer worth buying that isn't gas and get an actual foam cannon for it, but as a hobbyist I can't justify it unless I won a hefty scratch off or get a nice little bonus. If I were detailing for a living, I'd certainly get it both for richer foam, better clinging thus somewhat better cleaning, and the fact that it sells your service. I wouldn't be sold on a detailer using a Foamaster, but a Foam Cannon, sure would help seal the deal.