Denim bags look like a throwback and then quietly outlast everything else in your collection, because the fabric only gets better with knocks and scrapes rather than worse. A proper cotton canvas denim holds its shape without needing stiffening inside, and the colour fades in patches exactly where your hand rests on the strap or your keys sit in the bottom corner, so no two look the same after a year. That's the whole appeal to me. It stops being a bag you bought and starts being a bag you've actually used.
Fit is less about the size of the bag and more about the weight of the denim, because a thin, light wash will go baggy and shapeless within weeks, especially if you're the kind of person who carries a purse, phone, keys, lip balm and whatever else lands in there, and a heavier rigid denim holds its structure and its corners properly. Check the strap stitching before anything else, since that's where cheap versions give up first, usually at the point where it meets the body of the bag rather than along its length. If the hardware is painted rather than plated, expect the finish to wear through to dull metal within a few months of normal use, which looks tired fast. A well made one has proper rivets at the stress points, a lined interior so the raw denim doesn't shed fluff into your bag, and a zip that still closes smoothly when the bag is actually full rather than only when it's empty on a shelf. Avoid anything with decorative fraying at the edges unless you're prepared for that fraying to spread, because denim unravels if you let it.
Where it falls short of you is anywhere formal. Denim reads casual no matter how structured the shape or how smart the hardware, so it will always look slightly wrong against tailoring, against anything silky, against an outfit built for an occasion rather than a day. It clashes hardest with other heavy denim, a jacket or jeans in a similar wash will make the whole look flat and try too hard, so wear it with something that isn't denim at all if you're already in jeans. It's also not the bag for rain if it's unlined or untreated, because wet denim stays damp for hours and smells exactly like wet denim for just as long.
A good denim bag earns its scruffiness. A bad one just looks like it's given up.