I'll tell you what I tell anyone wavering in the changing room with one of these strapped on: forget whether it looks odd for a second and check what it does to your hands. If both of yours are suddenly free and the bag isn't swinging about or digging into your hip bone, that's the whole sale right there.
Fit is mostly about the strap and where it actually sits, not the size of the bag itself. You want it to sit on your hip or waist without flapping forward when you bend down, which means trying the adjustment properly in the shop rather than guessing. Too loose and it slides round to your back and becomes a small, expensive growth you can't see. Too tight and it cuts in if you're sitting down for any length of time, on a train or at a desk. Leather and leather look versions hold their shape nicely but take a while to soften, so the first few wears can feel a bit stiff and boxy against you. Nylon and the recycled technical fabrics are kinder from day one, they flex with your body, though they crease in a way that looks fine on a bag and slightly sad on, say, a jacket. Go for one with a decent buckle rather than a flimsy clip if you're planning to wear it daily, because the clip is always the first thing to go.
Where it falls short is anywhere you need to carry more than a phone, cards, keys and maybe lip balm. I have tried to force a paperback into one and it was not dignified for either of us. It is also not the bag for anything that wants to look pulled together and formal, an interview, a smart lunch, a wedding, because the whole point of it is a bit of sportiness and that doesn't dress down convincingly no matter how nice the leather is. And be careful with anything else slung across your body at the same time, a cross body bag or a bumbag worn higher up, because you end up with straps crossing over each other and looking like you're carrying luggage for a trip you're not taking.
Where it earns its spot is in an ordinary week. Jeans and a plain top, a simple dress with trainers, a coat with pockets that are already full of other things. It sits well against a flat sandal or a trainer, less well against anything delicate, since the practicality of the bag and the fussiness of a heel tend to argue with each other.
Wipe leather ones down with a dry cloth after a damp day rather than letting rain dry into the surface on its own.