It's honestly a personal preference. They used to tell people with back pain to get a firm mattress, but now they say that you should go with what's comfortable for you. Firmness provides support, but also creates pressure points because your body doesn't sink in to it, especially if you are a side-sleeper. Too soft of a mattress gives too little support, so your spine is out of alignment. Some people find that air mattresses are too soft, especially if they aren't pumped up enough.
If you find something that's comfortable, go with it. If you do buy a new mattress, make sure you lay on it for at least 15 minutes in the position you normally sleep in before you buy it.
(Also: use the box spring and frame that come with the mattress and use a mattress protector to prevent stains--if you don't, you can void the warranty. Even a tiny little water mark can be enough for the manufacturer to void it. Just FYI.)