I have tried many different mattresses in my day - from plush to pillow top to firm - and nothing can hold a candle to the IKEA Sultan Fjordgard mattress my husband and I bought about 6 months ago. We were hesitant to purchase from IKEA, knowing that sometimes the quality can be lacking there. But we went out on a limb after hearing about their 25-year guarantee. I can tell you that we have not been disappointed. The firmness is just right, and it has been very comfortable for us two back sleepers. I don't think it would be very comfortable for stomach sleepers, though, as I once woke up with a pretty bad backache after sleeping all night on my stomach for some reason.
The mattress itself has held up very well over the past 6 months - we have found no indentations from sleeping in the same spot too long and it is still just as firm as the day we brought it home.
I would caution one thing, however - the mattress is made of a very absorbent material. So if you are even slightly worried about spilling something on it or anything soaking through your sheets, I would highly consider some kind of mattress protector. We already have a few spots were things have leaked through, and the stains are difficult to get out of the mattress.