There's a particular kind of relief in climbing into bed after a long day and having the sheet actually fit the mattress, no bunching at the corners, no cold patch where the fitted sheet has popped off and you're lying on bare ticking at midnight. That's the whole case for sheet sets. You don't think about them until they're wrong, and then you think about nothing else.
Getting the size right sounds obvious until you're standing in a linen aisle realizing you don't know your mattress depth, and this trips up more people than you'd expect, especially in older houses with mattresses that run thick or pillow tops that add another few inches. Measure the depth before you buy anything, corner to corner, with a tape measure, not a guess, and if you're between sizes go up rather than down, because a fitted sheet that's too shallow will fight you every single night and lose. Standard depth works for most mattresses, but deep pocket sizing exists for a reason, and twelve inches of memory foam plus a topper is not a standard situation. The other mistake is buying based on thread count alone, as if it's the only number that matters. It isn't. Weave matters more, percale feels crisp and cool, sateen feels heavier and a little glossy, and neither is better, they just suit different sleepers and different seasons.
In an ordinary bedroom, a good sheet set sits quietly under whatever you've layered on top, a duvet, a quilt, a few pillows that don't all match, and it doesn't need to coordinate with anything else in the room because nobody sees it except you, which is honestly the most freeing thing about this entire category. Over time, cotton sheets soften in a way that's genuinely satisfying, almost like they're breaking in rather than wearing out, though that softening comes with some shrinkage, so wash new sheets before the first use and don't crank the dryer heat unless you enjoy refitting a sheet that now fits like a shower cap. Wash on cool or warm, skip the fabric softener because it coats the fibers and kills the breathability that made you buy cotton in the first place, and expect a good set to last several years if you're rotating between two sets rather than running one into the ground.
Where sheet sets fall short is if you run hot at night and bought something heavy and tightly woven expecting it to breathe, or if you've got a very deep or oddly shaped mattress and tried to save money on the wrong size instead of the wrong fabric. That's the one place I'd never cut the budget, because a sheet that won't stay on is a sheet you'll replace twice.