The first and foremost big reason why you should install a solar hot water heater is the money. The investment will be recovered in 3-5 years and then you will benefit from another 20-25 years of free energy. The second great advantage is the size of the water tank which tends to be double even triple than a standard water heater.
Now, with the disadvantages, the biggest one is the initial cost you will face, bigger than most of the other systems. Also you have to install the tank and panels on the roof, so you will need some extra hands. The next bad thing about this system is the fact that they are unattractive and occupy almost the entire roof space. The maintenance has to be applied from the roof and to end the list if there is no sun there is no heat, so you will need some boosters.
A great alternative to solar water heaters can be the heat pump system. This one is almost as energy efficient as the solar one but less costly and can be hidden. Their functionality is compared to the reverse process of the air conditioner, offering you hot water any instance. They are 3 times more energy efficient than traditional electric heaters and are a great choice if you decide to replace your old system.