homemade solar panels

[Jon]

Is there any advantage to building non-photovoltaic solar panels, for water heating?

I ask because a while back I had a rental house in the mountains, with glass solar panels for heating an old wooden hot tub. They were long since replaced by an electric heater, but they had not been removed. I believe it worked like this:

1. The panels sat in the sun on the hillside.
2. Water was pumped into the panels; it was heated in the sun.
3. Sun-heated water gravity-dropped from the panels into an insulated holding tank.

It's possible the system used some sort of electric assist (to move the water up, and possibly to push the temp up 10 degrees or so), but the crux was that the sun did the lion's share of the work.

Gimmicky? Does this sort of thing exist today? I'd love to fix a free broken Craigslist hot tub, but the main cost is the long-term electric, not the unit.