The question I get most about stretch jeans, usually from someone holding two pairs with the same label but completely different stretch percentages, is whether they'll bag out by lunchtime. The honest answer is some will and some won't, and the only way to tell is the pinch test: grab the fabric at the thigh, and if it springs back instantly rather than slowly, it'll hold its shape through a full day of sitting, walking and sitting again.
Worn regularly, a good pair settles rather than stretches out, which sounds like a technicality but isn't. The denim softens around the knees and hips within a few wears, moulding slightly to how you actually sit and move, and a decent elastane blend bounces back overnight rather than needing a wash to reset it. Where they do give up is at the knee after months of the same movements, a slight bag that never fully recovers, and that's just elastane fatigue, not a fault. Wash them inside out, cold, and skip the tumble dryer if you can, because heat is what kills stretch for good, it breaks down the elastane fibres faster than wear ever will. Hang them instead. I've had a cheap pair go papery and lose all their recovery after four hot washes, and a slightly pricier pair still bouncing back a year later because I was lazier about the dryer with those ones, which tells you most of what you need to know.
Where they let you down is anything structured. A blazer with proper shoulders and a stiff stretch jean together just looks like you got dressed in two different decades, one rigid and tailored, one soft and giving, and they don't sit well against each other. They're also not the ones for a long day on your feet in a formal setting, the kind where you need the leg to hold a crease and look the same at six as it did at nine, because stretch denim by its nature softens and creases through the day rather than staying sharp. And if you've ever had a pair that felt perfect in the changing room and alarming by early afternoon, that's not you, that's a blend with too much elastane and not enough recovery, and no amount of good posture fixes it.
Over time, the ones worth keeping get better rather than worse. The colour fades gently at the fold lines, the waistband stops feeling like it's negotiating with you, and the fabric that felt slightly stiff on day one ends up feeling like something closer to jersey. The ones not worth keeping just go thin and shapeless in exactly the places you don't want, and you'll know within a season which pair you've got.