I had one for probably about 20 years.
I still have it in storage, and will probably set it up again when I finish building our new master bedroom..
Which is to say, I loved it.
It does need to be heated, otherwise what
@Magna said will probably apply. Sleeping on them unheated is not pleasant.
But, I loved that you could turn it up in the winter when it's cold and stay cozy warm, and then turn it down in the summer so that it wicks away just enough of your heat so you don't feel too hot.
Also, I think the warmth of the bed had a great effect on healing any stiff/sore muscles/joints.
I sleep on my side most of the time, so I found the waterbed (with no baffles) the easiest to sleep on as my shoulders could sink in and the bed would conform to me. Most regular mattresses I just wake up with a sore shoulder.
We did have a few leaks over the years usually resulting in needing a replacement bag. But the frame stayed the same (although I actually rebuilt it from a double to a queen size after my wife entered the picture).
The leaks were an irritation when they happened, but I still always felt like the bed was worth it.
