I have serious back issues so finding a comfy mattress isn't easy.
We'd previously bought a Which best buy for about £1500, that was advertised as great for people with bad backs, but it was so uncomfortable I slept in the sofa for years as we couldn't afford to replace it.
We now have a Flaxby Nature's Finest 16,500 sprung mattress from Dreams. I spent months looking, trying out different mattresses for 30+ minutes at a time and researching, as I was scared to buy another uncomfortable one.
It was around £2,600 in 2018, but we got it reduced to £1,200 as 1 bottom corner had a small defect (kink/dip). One hlaf is soft, the other medium and it has a built in topper. It's been a very good mattress for my back, but I think the most comfortable one I've ever had was a latex mattress.
It's so heavy I can't lift it, though, and due to the topper it can only be rotated, not turned over.
It's been very comfortable, but recently my back aches by the morning, if I spent more than 7-8 hours in it. I'm 65kgs, bmi 25.
My dh is finding the same, although he is on the heavy side (100kgs) and had been 20+kgs heavier for most of the last 6 years.
We were advised to change after around 7 years, and it's 6 years old now, so that fits in with what we expected. I think being heavy means it doesn't last as long, too.