I wrote chain bags off for years as the thing that looks sharp on a hanger and then spends the day sliding off your shoulder in a meeting you're trying to look authoritative in. The chain was the problem, always too thin, always digging a groove into my collarbone by eleven o'clock. What changed my mind was a bag with a chain that had a proper flat link, woven through a bit of leather strap at the parts that actually sit on you. Suddenly it behaved. That's the whole trick, and nobody tells you that at the point of sale.
A good one starts with the chain itself, which should feel like it has some weight to it rather than looking like costume jewellery that's wandered off a dress. Pick it up and the links shouldn't rattle loose or catch on your sleeve. Underneath, the leather matters more than people assume, because a chain bag gets handled constantly, in and out of a locker, slung over a chair back, and a soft, cheap PU will crack at the corners within a season while a proper grained leather or a dense saffiano just softens with use. Check the stitching where the chain meets the body, that's the point that fails first, and it should be reinforced, not just a single row of thread holding the full weight of your phone, cards, and whatever you've thrown in for lunch.
On the styling side, this is where a chain bag quietly does more than a plain tote ever will. It dresses up a plain shirt and trousers without you having to think about it, and it's the one accessory that doesn't argue with a blazer, which matters more than it should when you're trying to look like you've got your life together by nine. I've worn the same cross body chain bag over a coat on a cold commute and then straight into client meetings with the strap shortened to sit at the hip, which is honestly the best test of whether the length adjusts properly. If it only comes in one fixed drop, think about where that sits on you before you buy, because shoulder height differs more than brands seem to account for, and a chain that sits wrong will pull on one side all day.
Size it for what you actually carry, not what looks neat on a shelf. A bag that forces you to carry a second bag has failed at its one job. The detail that makes a good one worth the money is a chain that's been designed to double as a short handle, twisted and held in the hand, no tugging, no pinching.