I would say these days they add value. Just make sure you will actually own the panels and they`re not under some sort of lease or rent-a-roof scheme. I talk to a lot of people who`ve had them fitted and they all say they`re great. It`s also worth considering what fuel your house uses - if you have mains gas for your heating you don`t benefit as much as if you use electric.
The other thing is the battery storage technology which is advancing apace and is very expensive now but is coming down in price all the time so you may want to find out if that will be compatible with the panels. If you have (or are considering) an electric car then battery storage would be a major benefit.