The thing nobody mentions about a denim jacket is that it gets better right around the point you think about giving up on it. Mine went through a stiff, slightly unloved phase for about a month, the cuffs didn't want to bend properly and it sat a bit square on the shoulders, and then one day it just softened into the shape of my arms and stopped feeling like outerwear and started feeling like a second layer of skin. Shops can't sell you that bit. You have to put the hours in first.
It earns its spot in spring and early autumn especially, those weeks when a coat is too much and nothing is also too much, thrown over a dress on the way to something outdoors or worn under a mac on a day that can't decide what it's doing. It does nothing for proper winter, the fabric just sits there being cold and smug, so don't expect it to replace a coat once the temperature actually drops. Before you buy, check the weight of the denim rather than the colour, because a thin one will go papery and lose its shape within a season, and have a proper look at the buttons too, cheap metal ones tarnish and go a sad brassy colour within a few washes. Check the stitching on the pocket corners while you're at it, that's usually the first place it gives up. Avoid wearing it with anything else in heavy denim unless you actually want to look like you're auditioning for something, a chambray shirt underneath is fine, a denim skirt alongside is not, and it tends to clash with anything already busy, so a loud print dress under it ends up looking like an argument rather than an outfit. For fit, go slightly cropped if you're petite or it swallows you, and size up rather than down if you're between sizes, since a denim jacket that's tight across the back will never relax the way the rest of it does, it'll just stay annoying forever. The sleeves are the other thing to watch, too long and they bunch unflatteringly at the wrist, which is a small detail that somehow makes the whole thing look cheaper than it is.
It's worth saying, it won't make an outfit look expensive, that's not really its job.
Thrown over the plain jersey dress you already own and were bored of, it suddenly looks like you meant to dress down on purpose.