Solar water heating systems, though not comparatively popular with regards to other types of heating systems for home use, are a very efficient solution which can provide great results depending on the circumstances of its use. The basic idea is really simple - a solar panel array accumulates the Sun's energy and uses it to heat up a container of water, which is then used in the heating of the household in traditional ways (radiators, floor heating pipes, etc). It's simple to install and doesn't require a whole lot of maintenance - and in the end, it provides energy that is more or less free to the end user!
The most noticeable difficulty related to using a solar water heating system is related to its installation - it can be a delicate and difficult procedure, but if it's performed by specialists, that shouldn't pose much of a problem to the user. There aren't many prerequisites for installing a solar water heating system in one's house - mainly, they must have the right to install it on the roof (e.g. not living in a shared household, or having the permission of the other occupants), and the room beneath the roof should be large enough to hold the boiler - as well as have a stable base, as a boiler filled with water can pose some hazards when placed on a weak floor.
Even though a solar water heating system can provide a great advantage in the way it utilizes solar power, it still holds one major disadvantage which makes it an unattractive choice for many - it's very dependent on the weather. For those living in areas where it's often hot, that's not a problem, but residents of areas with a highly variable weather see it as a not very effective solution. One way to overcome this is to store the accumulated solar energy and use it when the Sun isn't shining brightly enough, though this isn't a universal solution and is still dependent on the weather to some extent. Another problem with this type of system is that it requires more forethought than regular heating systems, as it takes a bit of time for the panels to properly heat up the water and make it usable for heating the house.
It should be noted that even though the idea of using solar panels is relatively new and only came around a few years/decades ago, there are known cases dating over a century ago of people covering their roofs with tar to attract heat rays and installing water boilers directly beneath - of course, this isn't as efficient as using solar panels, but it's still seen by many as the precursor to the modern solar water heating systems. Another curious phenomenon related to these systems is the demographic of their users - it appears that the vast majority of the world's solar water heating systems are used in China, which can be attributed to the low cost of manufacturing such systems there.