Earlier quoted context omitted.
I was happy with the two IKEA mattresses I bought. I have one super-cheap foam one that I used for many years and keep around for guests; guests say they like it more than their mattress at home, and I think it only cost $30 and has lasted going on 15 years at this point. I also had one of their more expensive latex foam mattresses, and that kind of developed a sunken in spot in the middle where I slept on it after ~…
One caveat about IKEA mattresses: being a multi-national company, verify the mattresses and linens you're buying complies to the same sizing in your region of the world; they can differ and there's always stories about how IKEA sheets don't fit standard sizes or fit them well, at least.
In all three of those countries, Ikea sells mattresses in the regular local sizes. I recall that, maybe 20 years ago, Ikea in the UK only sold beds and mattresses in their native (Swedish? Euro?) sizes. But, in the last few years, I've bought both beds and mattresses from Ikea that are standard UK and standard US sizes.
Of course, it's always good to double check the actual measurements of your old/new mattress before pulling the trigger.