I've used these things for 7-8 years now - they're great as long as you recognize limitations. Used one for several years as my "lunch bag" - holds a good size cutting board, Tupperware with various silverware & a few spices, roll of paper towels in a gallon ziplock, small container of hand sanitizer, etc. I rig it on top of my load, behind where I sit on boat, (unashamed to admit usually just carabinered in) and it has survived major hits on Grand and other trips. Usually get 2-3 years out of a cube, it's UV that does them in, the zipper will separate from the fabric and then it's toast.
Can also rig below decks and they are "splash-proof" but not "submerge-proof." Great for organizing food on long multi-days. I also use one to carry drysuit/rain gear, again just 'binered into load behind me, easy to get to and open.
If you want to use to carry stuff up to camp, one in each hand w/carry handles will hold as much as I've ever wanted to lift. But if you're going for car camping or expect to use in the carry bag, stay with the 3 unit, much easier to manuever and will hold as much as you ever need. I have both the 3 and 4 cube carry bags, the 4 unit one has lived in garage forever... the three bay one we use infrequently for car camping. Mostly the cubes are just handy to organize stuff and totally rainproof except in gale force winds - then you get a drop or two past the zippers, which have tunnel protectors.