* I wonder how the cube works and how much it weighs?
It looks like a big plastic bag in a box to me. Figure water weighs about 8lbs./gallon. Fixer is heavier, maybe about 9 lbs./gallon. Figure 45 - 50 lbs. including the cardboard box. Shipping from B&H to my address is almost $34, and that's only 20 miles away.
* I suppose you don't have to mix all at once? Does anyone use these?
No there's no need to mix it all at once. In that regard it's no different from any other rapid fixer concentrate. It makes 20 gallons of film strength working solution, 40 gallons at paper strength.
* The 5 gallon size must be heavy and awkward. Where do you put the cube and how
Beats me, but I could find someplace to stash it.
A better deal is Kodak Flexicolor Fixer and Replenisher. Shipping costs are about the same, but the fixer is half the cost. I use it the same as any other rapid fixer. Dilute it as directed for film, double the dilution for papers. Works as well as or better than rapid fixers sold for B&W. Been using the stuff for years now on almost everything with no problems at all. See this link:
http://www.bhphotovideo.com/c/product/27599-REG/Kodak_1597392_Flexicolor_C_41_Fixer_.html
Obviously, if a hardening fixer is required this isn't the best choice. For the rare times when I do need a hardening fixer, I use plain old Kodak Fixer in the yellow bag. It's not rapid and it's sold as a dry powder, but it works and is inexpensive.