Carrying cases are a real issue for designers. There's a competition between utility and capacity at play, it can't be easy to understand what would really appeal to a user.
I'm a fringe case, my interest is primarily in painting and collecting models. My wife is uncomfortable with the idea of having them on display, so I keep models in carrying cases so they can be stored conveniently. Also, I sometimes need to transport armies locally / over large distances in North America and Europe. So being able to organize models, retrieve the right ones at the right time, and know that they won't break along the way is important to me.
My armies are stored in carrying cases. I use different solutions for infantry and tanks. Currently, I'm using 7 of the following for storing miniatures:
Each one has 6 foam inserts that store 36 models each for a total of 216 models per case. 216 may sound like a large number, but that's low for the way many people play the game. My cases are filled to capacity and I need to use smaller cases to deal with the overflow, which creates a secondary issue for storing the cases themselves. Since I end up with 2 sizes, I can't have a single stack where they all reside.
As far as the foam inserts go, they are sufficient for storing a standard game model. Where they fail is with larger models and detail pieces. For example, I have a Grey Knights army that has larger "Terminator" models with books attached at the top of the neck area. The Terminators don't fit into the standard slots and the books come off when I do.
There's 2 ways I've addressed this problem: by altering the foam trays and by purchasing a different carrying case. For the former, I don't like altering foam trays. It's possible to rip part of the foam tray out to allow larger models to be stored, but this causes the trays to degrade faster and looks ugly.
The other case I bought is a Crusade carrying case. It's the current version used by Games Workshop.
What I liked about it is the collapsing foam and the carrying capacity. It promised to hold about the same number of figures as the ones I currently used, plus it has this collapsible foam that is supposed to be gentler on the figures. In practice, the foam still knocks those books off the Terminators shoulders, and I've never been able to fit 216 models into one. It could be that I'm using it wrong, but it's a carrying case - it's not supposed to require a lot of figuring out to work.
Something that can address the problem of variable size miniatures would be valuable. Not really a fan of Games Workshop's transport systems, feels like the design skews heavily towards miniatures of a specific size / shape and without adjusting effectively for others.
For tanks and larger models, I have been using these things from SnapWare.
It's easy to understand how they work, there's a couple latches on each tray. You literally snap them on and off to add more trays.
The inserts you see in the picture are made of cardboard, I threw them out and replaced them with high-density foam and styrene to protect the models in transport. On several of the trays, I added a thin sheet of metal on the bottom to allow me to keep miniatures in place with magnets.
What I like about them is the modularity. I can add a new tray whenever I want to increase the carrying capacity. It stops me from having to buy new carrying cases because each tray attaches to another. I also like the fact I can customize it. I have slots for specific tank variants and know that details won't break off. The whole system is durable, it fell out of a car moving about 15 mph once and the case + everything inside survived.
But what's really been cool is the fact the snap fit stuff can also serve as terrain for games. I like that I can pull out my army have a box that's just over a square inch to put on the table, and that I can control the height of the building. Many of my armies rely on mechanics that require line of sight blocking terrain, and one of the things gamers often lack is a personal collection of scenery. When a carrying case can be more than a carrying case, that's pretty exciting.