Sofa cushions have a reputation for being the thing that makes a cheap couch look expensive, and most of the time that reputation holds up. A good set of covers and the right fill can rescue a sofa that's otherwise seen better days, and I've watched a plain beige three seater get complimented purely because someone piled four decent cushions on it. The flip side is that people expect them to fix structural problems they can't fix, and that's where the disappointment creeps in.
Where they genuinely fall short is on anything that actually needs support, a reading chair or a spot where someone naps for real, because the soft, squashy ones that look so good in photos flatten out fast and stop doing their job within a season. If you need back support, you want a firmer insert, and no amount of styling covers for that. On color, I've learned to be suspicious of anything too matched to the sofa itself, since it reads flat and a bit showroom. Mixing two or three tones that pull from the rest of the room, a rug, curtains, even a painting, does more work than a cushion that's trying to blend in. Cream, rust, and deep green have aged well in most of the rooms I've seen them in, and they shift mood nicely as the light changes, warm and soft in the morning, richer once lamps come on at night. Pure white is lovely for about three weeks and then it isn't, unless nobody in the house eats near the couch, which in my experience is nobody's house.
The difference between a cushion that holds its shape and one that goes sad and lumpy within a year usually comes down to the fill and the stitching, not the fabric print everyone gets distracted by. A feather and down blend with a proper inner liner will outlast a cheap polyester fill by years, and you can tell a well made one by the piping, which should sit flat and even, not puckered at the corners. Removable covers with a real zipper, not a stiff plastic one that jams after two washes, make the difference between something you keep and something you quietly throw out. These suit almost any living room, but they do the most good in homes with a lot of daylight and plain furniture, since the light shows off texture and color far more than it shows off shape.
Fluff and rotate them weekly so the fill doesn't settle into one flat shape on one side of the sofa.