The question I get most is whether it's worth buying new covers or just buying new pillows, and the answer is covers, every time, unless the insert itself is actually shot. A cover is the part that touches everything, the part that fades and stains and gets slept on, and swapping it costs a fraction of replacing the whole pillow. I treat the insert like the frame and the cover like the finish, and only one of those needs to change when I get bored or the color stops working.
These make the most sense in rooms that get used hard, a living room with a dog that thinks the couch is his, a bedroom that doubles as a reading spot on a Sunday. In a rental, they're close to the whole game, since you can change the feel of a room without touching the walls or asking anyone's permission. I go heavier and softer in winter, something with a bit of texture that holds warmth, and swap to a lighter woven cotton or linen for summer, mostly because the thicker ones can feel like sitting against a radiator by August.
For color, I've learned to stop trying to match the sofa exactly and just pick something that sits a shade or two off it, warmer or cooler, so it reads as chosen rather than matched by a computer. Solid, muted tones wear the best over time, a soft clay, a faded green, an oatmeal that hides a coffee spill better than white ever will. I bought a set of bright mustard ones once because they looked great in a lamp lit photo, and in my actual living room they looked like a warning label. Busy patterns are fun for a season but they fight with everything else in the room faster than you'd expect.
Where they fall short is with anything delicate that needs babying, silk or a cover with a zipper so fussy you need both hands and a swear word to get the insert back in. If you want something that looks the same after being thrown on the floor by a kid or a dog fifty times a week, skip the fragile fabrics and the covers with hidden zippers that jam, because you'll stop using them within a month out of sheer annoyance.
Next time you're holding one, skip the fabric first and go straight to the zipper, run it the whole length and feel whether it catches, because a cover you dread opening is a cover that never gets washed.