My dad's had one for years.
Slept on it once, had a really, really good night's sleep!
Shame you need either a ground-floor bedroom, or a very well built house to take the weight!
Couldn't have one in my current house (7 years old), where the particle-board "floorboards" creak every time even a baby crawls over them! A water bed would end up in the living room after the first night, at a guess!