A bookcase or a shelf is the most honest piece of furniture you can own, because within a week it stops being furniture and just becomes a record of what you own and how much of it. I think that's why a badly made one bothers me more than a badly made sofa. A sofa just sits there. A sagging shelf actually fails, visibly, with your books tipping sideways into a curve like it's trying to apologize.
The difference between a good one and a disappointing one usually comes down to the shelf itself, not the frame around it. Thin particleboard will bow under books inside a year, no matter how solid the sides look, so press on the middle of a shelf before you buy anything and see if it flexes. Solid wood or a proper plywood holds flat, especially if the depth is at least ten inches and there's some kind of back panel or bracing keeping the whole thing from racking sideways when you lean on it. On a bookcase, look at how the shelves meet the sides: dadoes or proper joinery will outlast your books, while shelves just resting on little pegs or cam locks will eventually work loose. For wall mounted shelves, check what's actually holding them up, because a bracket rated for the picture it's screwed through is not rated for a stack of hardcovers. Measure the wall studs, not just the wall, before you commit to a spot. For a freestanding piece, measure the room first, not the gap you think looks right. A bookcase that's taller than your eye line will dominate a small room and make the ceiling feel lower, while a run of shelving that's too shallow for what you actually own just becomes a dumping ground for things stacked two deep, where you can never find anything at the back.
Where this goes wrong is when people buy a tall, heavy unit for a room that needed something lower and open, or hang deep shelves in a small kitchen where they end up catching your head on the way to the stove. Open shelving also shows everything, dust included, so it's the wrong call if you're not willing to keep it tidy. It's at its best in a room with wall space to spare and things worth looking at, a living room, a home office, a hallway that needed a reason to stop being just a hallway. Garages and utility rooms want something sturdier and plainer, built for weight rather than looks, and that's fine too, different job entirely.
Whatever room it goes in, it gives the things you already own somewhere proper to sit, instead of stacked on the floor pretending that's temporary.