Had one for 30 years, and love it. The "mattress" and heating unit was replaced a few years ago, after the first had finally passed its use-by date. It is a waveless. Among the reasons I like it are that as bpas105 said you can have a warm bed in winter and turn it down in the summer. I also find it a lot easier to get a good night's sleep - much more comfortable.
Finally, they don't soak up body fluids and skin flakes or harbour mites and mould and such, which is particularly good for allergies and general hygiene.
