The question I get asked most is whether a bag is worth buying if it can only really do one job, and my honest answer is no, not unless you have an unusually large bag budget. The ones that earn their space are the ones that can carry a laptop on a Tuesday and then go out on a Friday without you feeling like you brought your office with you.
In an ordinary wardrobe, a bag has to sit against a lot of different outfits, so I think about what it touches rather than what it matches. A structured bag in a neutral colour will sit happily against tailoring and against jeans. A soft shoulder bag softens anything too stiff. A woven bag does a brilliant job of making plain trousers and a plain top look like you tried, without you actually trying. I tend to buy one bag that can take weight, a work bag or a bigger leather one, and one small one that forces me to leave half my life at home, and between those two I cover most days.
Before I buy, I check the hardware first, because that is where the cheap ones show themselves. I pull the zip the full length twice, feel for any snagging, and have a proper look at the lining seams, since a lining that is already puckering in the shop will be worse after three months of being thrown onto a car seat. I check the stitching on the strap join, since that is the point that takes all the strain, and I look at the base, because a bag with no structure there will slump the moment you put a water bottle in it.
Getting the size right is less about measurements and more about what you actually carry, so I take my usual bag contents into the shop in a separate bag and tip them in before deciding. Straps are where most of the disappointment lives. A crossbody strap that looks fine on the hanger can sit awkwardly short or long once it is on your actual body, so adjust it while it is loaded, not empty. Shoulder bags should stay up on the shoulder through a proper arm swing, not just standing still, and if it slides off after two steps that is not going to improve with wear.
How a bag ages tells you a lot about whether it was worth it. Good leather will darken slightly where your hand rests and soften at the corners, which looks earned rather than tired. Woven bags relax and lose their stiffness, which usually suits them. The cheaper fabrics and coated materials tend to crack along the fold lines instead, and once that starts it does not stop, so if you are choosing between two similar looking bags, that is the detail worth paying attention to.