Rigid denim suits anyone who wants jeans to hold their shape rather than move with every sit down and stand up, and who doesn't mind a short break in period before they feel like yours. If you need something to be comfortable from the first wear, or you're between sizes and hate the idea of fabric that won't give you an inch either way, this isn't the pair for that week. Give it a fortnight though, and it rewards patience in a way stretch denim never quite does.
The first few wears are the test. Rigid denim is stiff enough that it can feel like wearing cardboard round the thighs, and the first time you sit down at a desk you'll wonder if you've made a mistake. By about week three, mine had softened at the knee and the back of the thigh, exactly where I bend, and nowhere else, which is the whole point. Wash it inside out, on a cold cycle, as little as possible, because every wash strips a bit of that stiffness and the fading you get from actually living in them is better than the fading a machine gives you. Hang dry. Tumble drying rigid denim is how you end up with jeans two inches shorter and a waistband that's given up.
The difference between a pair that ages well and one that just goes baggy and sad is mostly in the weight and the weave. A proper rigid jean has some heft to it, you should feel it when you pick it up, and the denim should feel tightly woven rather than papery when you rub it between your fingers. Check the inside of the waistband and the pocket bags, if they're flimsy and thin there, the whole thing will give up early no matter how solid the front looks. Rivets and the button should feel properly set, not the kind that spin loose after a month. A decent hem finish matters too, a chain stitched hem sits better and doesn't go wavy, though you'll rarely know which you've got until you've worn them a while.
For colours, a mid to dark indigo is the easiest thing to own because it hides the first stage of fading and still looks deliberate when it does start to go. Black rigid denim looks sharp on day one and is the first to look tired, it greys out fast and never quite recovers. Any very light or whitened wash shows every crease and crock mark early, which some people love for the worn in look and some people find just looks like a mark you can't get out.
Next time you're holding a pair, run your thumb inside the waistband and the pocket bag before anything else, that's where the cheap ones give themselves away first.