Like UNG said, I'd check the small print.
Any firm fitting solar panels to your house for free should let you keep ALL the free electricity - the electricity that is sent back on to the grid may be a different issue. They should receive FITs (the government subsidy for electricity generated) and no more. Maintenance should be their responsibility, and comfortingly, in their best interests. If the system isn't working as well as it could, it would have more of an impact on the installing company than the customer.
The awkward part comes from when you want to move house. Will it add value to your house? I'd say it probably would, but I can't be sure. Will some potential buyers pull out of a deal because they think they look ugly? All of these firms will have it written into the contract that you cannot remove the panels from your home - unless you pay them a high fee, I would imagine.
Is it too good to be true? Depends what you want from it. If you want some free electricity which will save you around £200-300 a year then there really is no snag. However, if you can, and I cannot urge this enough - you should pay for the install yourself. It IS expensive but the returns from the system are very attractive (just ask yourself why these firms are so keen to install them for nothing in the first place). If you paid, for example, 12k for a 3kW system, you could expect returns of (depending on where you live) around £1300-1400 a year. You will quickly make your money back.
So in my opinion, the only real snag is that if you have them fitted for free you just won't be earning as much money as you could be earning.
Go and speak to your bank and get them fitted yourself
There are no shortage of firms ready to do the installation for you.
