Sleeper sofas are a great way to dual purpose a room for the occasional guest. The most important thing to remember when shopping for a sleeper sofa is to try it out. Open it up on the sales floor and lie down. If you are not comfortable, then your guests won't be either. Many retailers now offer mattress upgrades for their sleeper sofas.
If your sleeper will be getting frequent use, an upgrade is worth the extra money. Be sure to open up the sleeper yourself. Chances are if you can't easily get it open in the store, you won't be able to easily get it open when an overnight guest arrives.
There are also some great new options on the market for sleepers in addition to traditional sleepers. New modern designs combine European styling with a futon's functionality. Backs and arms lower to create a sleep surface, instead of a mattress being folded up inside a sofa frame.
Some manufacturers now offer air beds that work with your sofa: just take off the seat cushions and slip in the air mattress for a raised bed option. It takes up less space than a regular airbed and a traditional sofa in the same room. This style of sleeper is easier to move than conventional sleepers since they do not require a heavy folding metal mechanism.
Whichever option you choose be sure to measure the room and the doorway before you go shopping. Not only do you need the room for the day-to-day sofa, but you need to have the room to open the sleeper up for use as a bed. Measuring the doorway will let you know if you can get the sofa into the room.