Particularly popular in the 1970's and 80's, waterbeds are still fairly popular today thanks to a few unique features. The most notable advantage of a waterbed is its ability to be easily (and safely) heated. Those living in colder climates can certainly understand how pleasant it is to climb inside a nice warm bed in the winter.

To be fair, water beds have changed quite a bit since 1975 when you rolled around on your aunt and uncle's waterbed as though you were tossing and turning in a tropical storm. Modern waterbeds come in two types, soft-sided and hard-sided. The classic waterbed is the hard-sided variety with sturdy wood sides helping to keep the water mattress rigid (and contain spills in the event of a puncture).

Updated waterbeds fall into the soft-sided category, a hybrid of a conventional mattress and a water bed. These mattresses essentially have water chambers inside a foam and fabric mattress casing. Water beds also happen to be very good for people with allergies as the outside of the bed can be wiped down to make sure no mites or pollen are present.  Additionally, waterbeds have a long lifespan, typically well over ten years. 

Water beds were first invented over 100 years ago but didn't gain widespread popularity until 1968 when a design student at San Francisco State University used modern materials to make the waterbed a practical reality. Charles Hall created the basis for the modern water bed but wasn't able to secure a patent for his work due to several instances of prior art dating back to 1871.

Unfortunately, heating a water bed can be quite expensive. A bigger bed is obviously more difficult to heat but new low-wattage heaters can help keep costs down.  Still, expect to pay up to $500 per year to heat a water bed. You must also be very careful to put the water bed in an area that can sustain a lot of weight. Putting a water bed in the middle of a large upstairs room can be a bad idea.

Most mattress chains don't sell water beds.  You're better off going to a local furniture store.