It makes green eco sense as you are saving CO2 emissions, though the economics are somewhat more complex.
There is a really good independent article here: http://www.yougen.co.uk/blog-entry/2635/How+much+does+solar+thermal+really+save’3F/
In essence if you have a high cost heating system – Direct Electric, Electric Flow Boiler, Storage Radiators and Immersion, or LPG, then yes it makes economic sense.
If you are on oil or mains gas then the answer is not so simple, the above article suggest savings of up to £75 / year, you then need to add in £50 / year maintenance costs giving a net saving of £25 / year.
So, if you are on mains gas or oil, you will invest in solar thermal because reducing your CO2 emissions is important to you, similarly if you are a heat pump or biomass user, then it will be part of your whole emissions reductions profile.
If you are on electric or lpg, then not only will you be doing something significant for the environment, you will also make a big impact on the actual costs of producing your hot water.