This suits bedrooms that currently have nothing on the walls except the mirror and maybe a calendar, and it suits people who want the room to feel finished without a big renovation. If your walls are already busy with shelving, a gallery of family photos, or a headboard with a lot going on in terms of pattern, think twice before adding more. A bedroom can hold quiet and still, or it can hold one good piece of art, but trying to do both at once usually just looks crowded.
Measuring up matters more here than in almost any other room, because the bed takes up so much visual weight that the art has to hold its own against it. A rough rule I use: the piece above the headboard should be somewhere between two thirds and three quarters the width of the bed, not the width of the wall. Too narrow and it looks like an afterthought floating above the pillows. Too wide and it starts fighting the nightstands on either side. Hang it so the center sits around fifty seven to sixty inches from the floor, lower than you think, and leave six to ten inches of air between the top of the headboard and the bottom of the frame. If you've got nothing on the walls at all right now, start with one piece over the bed before you think about anything else, a single frame big enough to matter, rather than three small ones that need careful spacing you'll probably get wrong on the first try. A cluster is a second purchase, not a first one. Where it goes wrong: a small print over a king bed, hung too high because someone was worried about knocking it with a pillow. It ends up looking like it drifted up there on its own.
It does not sit well opposite a window with strong morning light hitting it directly, glass will throw a glare that makes the whole thing unreadable from the bed. It also struggles next to heavily patterned wallpaper or a busy duvet, where the eye has nowhere to rest. And skip anything with a glossy white mat and a thin metal frame if the rest of the room is warm wood and soft fabric, it reads cold and a little like a hotel corridor.
The detail that makes a good one worth living with is the quiet of it at night, when the lamp's off and the room's dark except for a strip of hallway light under the door. A piece with real depth to the color still holds its shape then, instead of going flat and forgettable the second the main light's off.