This is a REALLY firm mattress, like sleeping on a floor. That is just what I wanted.I have a genetic condition called Ehlers Danlos Syndrome, which means my skeleton is not held in place properly due to my connective tissue being 'floppy'. This is the best mattress I have ever owned (far better than 'luxury' brands costing upwards of £1,000) and supports my skeleton perfectly. At last, I can have a comfortable night's sleep and not wake up with dislocated joints (yes, that does happen) and severe pain all over my body.So, for me, this mattress is heaven itself.However, you have to realise that this is like sleeping on a breeze block for most of your body. If your current mattress is saggy, it is probably because it's old or worn or hasn't been turned properly. Do not buy a 'very firm' mattress to replace it unless you want a really HARD bed! Pretty much any new mattress will be firmer than an old one. This mattress is designed for people who are obese or disabled or have an extremely bad back that is permanent. I would imagine that for other people it would be way too firm. So beware of that before you buy.But if you know you want a very firm mattress, then believe me, you can't do better than this for the price. So long as you rotate it every week initially as recommended (after two months you can rotate it once a month), it does not sag. I have had mine about six weeks now and there is no sag whatsoever.