The moment that sells it is waking up without doing the mental inventory of which part of your body hurts. No shoulder dig, no hip ache, no lying there doing the math on whether it is the mattress or just getting older. A good one disappears under you. You stop noticing it, which is the whole point, and that is a strange thing to pay a lot of money for but there it is.
The honest limits first: a mattress cannot fix a bed frame with no real support, and it will not solve a room that is too small for the size you actually need, it will just make the problem softer under you. If you sleep hot, some foams will work against you no matter how good the reviews are, and if you share a bed with someone who moves a lot at night, a mattress with poor motion isolation will cost you both sleep regardless of how nice it looks in a photo someone sent you. A queen feels generous in a bedroom with ten feet of width to spare and feels like a boat in a room where you can only approach it from one side, so measure the path to the bed, not just the bed itself, before you commit to a size that seems reasonable on paper. Firmer mattresses tend to suit side and back sleepers who need their spine kept level, while softer ones suit stomach sleepers less and lighter people more, and this is one case where what your friend loves might be wrong for you entirely. A mattress bought for guest use in a spare room does not need the same investment as the one you sleep on every night, and treating those two purchases the same is a quiet way to overspend. Where it goes wrong is in rooms with low ceilings and a frame with a tall headboard, where a thick mattress on top makes the whole thing feel like you need a step stool, and in rentals with original hardwood or herringbone floors, where a heavy, unwieldy mattress is a pain to maneuver in and just as much of a pain to get out again later. What separates a mattress that lasts from one that sags by year three usually comes down to what you cannot always see at the showroom, the quality and density of the foam layers, whether the coils are individually wrapped or one connected unit, how the edges are reinforced so you are not sliding off when you sit to put your shoes on, and whether the cover is a real quilted fabric or a thin wrap that pills within a year.
Next to it, put a bed frame sturdy enough to earn the mattress you chose, and a couple of pillows in the firmness that actually matches how you sleep, not the ones that looked nicest folded on the display.