- Trying to help your back (it helps for some, for some it doesn't)
- Have limited space in your house or apartment
- You like to travel light and often
- Can't stretch comfortably on the couch
- Don't have the money to buy a mattress
- Stay cooler during the summer
- Just want to try something different
Sleeping pad with sleeping bag.
Mostly because of convenience, my preferred way of sleeping on the floor is on a light and comfortable sleeping bag on top of a sleeping pad. It's softer than the floor, but still firm. If you sleep well on your stomach and your back, this works great, but it is harder on my shoulders when I sleep on my side. Because of this, you may have to turn a few times throughout the night, which to me is normal. I ultimately preferred this way the best because it takes only a minute to "make the bed". Fold the sleeping bag a couple of times, roll up the pad, and you're done. Here is the sleeping pad and sleeping bag I use.
I vouch for the Slumberjack, but if it's unavailable as you read this, there's plenty of other cheap sleeping bags here.
Cots are comfortable. Their advantage is that they get you up off the ground, which is a plus if you've got a draft in the house during the winter. To me the biggest disadvantage with a cot is setting it up, taking it down, and stuffing it back into its bag every day. It only takes 5 minutes, but it gets old after a while. The cot I have is a Field and Stream. It makes a lot of noise when I shift around, but like I said, it's comfortable. My brother had the exact one too, but his started sagging in the middle and he couldn't use it anymore. I suspect it was because he would sit on it, which isn't a great idea on a stretched canvas surface. I distributed my weight evenly when I got up and down and I haven't had any problems with sagging (I used it nonstop for maybe 3 or 4 months). The only problem I had with it is that its carrying bag tore on me.
I've tried two different air mattresses, a Coleman and some other one. Both were self-inflating, which means there is a built-in pump that you just plug into a wall outlet. Whenever it gets low on air, you just turn it on until it's firm again. They also have a release valve to let the air out. To me, air mattresses are very comfortable, just as comfortable as a bed even. A downside is what to do with it during the day if you don't have an extra room. It's not convenient deflating and inflating it every day, so an alternative is to leave it standing up against a wall, which is kind of ugly. I actually would have stuck with the air mattress but both of them were punctured by my cat! The little bugger likes to get my attention by clawing at it in the middle of the night. So if you have pets, keep that in mind. Also, just like the cot, it's probably not a good idea to sit on an air mattress, as you put undo stress on one portion, eventually popping or tearing it. When you get up and down off of it, distribute your weight evenly.
Again, I vouch for the Coleman, but there are [cheaper] alternatives here.