We use a 4" hydra-assist valve on our 4" supply lines for every fire. There is minimal friction loss and it only weights 35lbs. When you need an increase in water pressure from the next engine company on scene, it works well to have the valve already in place.
We have looked at the Kochek assist valve and the Humat Valve.
We have adequate spacing that we don't use hydrant assist valves. We use split 5" LDH lays if required and sometimes lay in dry with the 2nd-due engine relaying without a hydrant assist valve.
For relays or direct hydrant connections, we sometimes use a 2.5 inch gate valve on one of the 2.5 inch hydrant discharge ports and later add a 2.5 inch line to the auxiliary intake, but the need for that is rare with our water system.
We have been using the hydra-assist valve for 20 years made by Snaptite.
I am aware that many departments will defend their hydrant pressures, hydrant spacing, LDH hose, sufficient apparatus/manpower, response times, etc for not trying the hydra-assist valve. The question that has to be asked is, what if there is a water system failure? Terrorism, natural disaster, human error, delayed response, conflagration, etc. If water pressure can not be increased and personnel are not routinely using the equipment when the time comes, can resources recover from that kind of pressure loss?
What possible difference will a hydra-assist valve make if you have a water sytem failure? Is the enginge that hooks to the valve going to manufacture water to supply to the attack engine?
When planning for a water system failure, you need to carry hard sleeve/suction hose and a map of your static water sources, not a hydrant assist valve.
Our entire system (FD, water system, FD water supply practices) are designed to fit together. We have excellent response times, excellent 2nd-due response times, a good water system with excellent hydrant density, and the ability to use an alternate, non-potable hydrant system in key parts of our response area.
We have had single hydrant failures, but with hydrant spacing typically at 400 feet, we are rarely more than 4 sections of LDH from a 2nd-due water supply source.
You might want to give TFT a look. They have a new assist valve. I've seen it in Fire Apparatus Magazine and watched the videos. Might be another one to consider.