Hogue pistol grip storage pods are terrible to install and uninstall, at least on the AK.
FABs are mediocre but functional.
Magpuls are well done but their grip storage pod makes for a rather wide grip (ARs do require more spare parts on hand in my experience than blowback carbines)...
BCMs have my favorite hinge cap design...
Pistol grip storage is great as it keeps critical parts, etc, on the gun, and given the grip location puts that weight at a great balance point for the rifle... Basically the only cost to the shooter for having grip storage is any excess initial dollars cost, and then a couple ounces in weight. All things considered totally worth it.
Great for
-critical spare parts kit such as firing pin, extractor, takedown pins, etc -- stuff likely (likelier) to break or get lost and cease weapon function
-optic batteries
-boresnake or other basic field cleaning kits
-specialty ammo (snakeshot, etc)
A rubbery closed cell foam might work well and relatively cheaply for grip insert organization.
Bottom line, I see grip storage as something like an add on during home construction -- you can pay a little extra upfront to do it initially; otherwise, on the backend you basically have to start over from scratch.