Although they sound like a good idea, they have their shortcomings. The main one is that you need to lift them into and out of the trunk of the car or truck bed, and drag them to the boat. A 30G Gas Caddy would weigh 250# plus, when full.
I have a 14G Gas caddy. I use it to keep my speedboat topped off, as it only has an 18G tank. It was the best birthday present I'v ever received from wifey, so I try to use it. I would have been just as happy with another tie.
I have abut 10 Jerry cans of 5 and 6 Gal each. When I fill my 21 footer, I use 4 or more of them. The gas caddy would take multiple trips.