Leather dresses have a reputation for being a bit much, something you'd wear to be looked at rather than to actually live in. I think that's unfair. A good one is one of the easiest things to throw on when you can't be bothered to think, because it already looks finished before you've done anything else to it. The bad ones earn the reputation, though, and there are a lot of bad ones out there.
Where it's the wrong choice is anywhere you'll be sitting for hours or moving around a lot. A stiff faux leather dress on a long car journey or a wedding with a sit down meal is a genuinely uncomfortable few hours, and real leather in heat just feels like being wrapped in cling film. It's also not forgiving if you're self conscious about clinging, since both real and fake leather show every lump and seam underneath, so the usual rules about what you wear beneath a close fitting dress apply twice as hard here.
Fit is where most people go wrong with these. Leather, real or fake, does not have the give of jersey or ponte, so sizing up slightly is usually the safer move, especially around the hips and upper arms where a snug fit turns into a genuinely restrictive one by the end of the day. Try sitting down in the changing room, not just standing in front of the mirror, because that's when a too small leather dress tells on itself, usually by riding up or pulling tight across the front in a way that won't settle. Watch the length too. A leather dress that's a sensible knee length standing up can look several inches shorter once you sit, which matters more with this fabric than almost any other.
The difference between a decent one and a disappointing one is mostly in the hand feel and the seams. Cheap faux leather is thin, goes a bit plasticky and shiny in warm rooms, and the seams pucker rather than lying flat. A better one, leather or a good quality alternative, has some weight and a slight matte softness, and the zip is sturdy rather than the flimsy metal that catches on the lining after two wears. Check the lining itself, because an unlined cheap leather dress will stick to tights or bare skin in a way that gets old fast.
It clashes with anything else that's trying equally hard, so skip pairing it with chunky statement boots and heavy jewellery all at once, and be careful layering it under another leather or patent piece, which just looks costumey rather than put together.
Reach for it when you want to look deliberate with almost no effort, an evening out, a dinner, anything where you're mostly standing or sitting somewhere comfortable rather than on your feet all day.