I have solar hot water (a self built system) and have installed a few systems. All work extremely well, the best solar investment you can make!

One customer of mine has a system he also uses to heat his house - I only installed it this spring so as yet we don't know the results.

I'm currently working on a project to install a hybrid solar/heat pump heating system for my house, again a home made system hopefully I'll have it installed soon and will know how well it works this winter. I had the heat pump working last winter and saved in the order of 1500 euros in heating oil by running it.

I must admit to not being a fan of the gravity fed systems as I've seen problems such as boiling/calcium build up etc. with them. The system I prefer is a closed solar circuit with circulating pump and an indirect heated tank.

Mine, for example, is a single panel (about 2.4m x 1.2m) and a 160 litre water tank. We have hot water from this pretty much from late February to November. Outside of this I use a combination of a heat pump or immersion heater to heat the water, the former being the most efficient. The pump and control runs from solar so effectively costs nothing to run.