I foamed for the first time with the Gilmore gun I picked up on Amazon.
In summary, my impression is that it did save me time, but it also increased the amount of soap I used.
I can't say with certainty that the car is any cleaner for it, but the initial foaming plus rinse down did take away some visible dirt. Is this, coupled with the 3 minutes saved worth the cost of the gun, materials, and water? For me, I would say yes, but its borderline in its value. Therefore if you are someone on the fence about getting one I'd recommend not; however, if it looks interesting to you, you may want to give it a shot.
For reference, I used cheap RainX shampoo which I buy for $5 for 64 oz. Biodegradable and foams well.