The way these people make money is by selling the excess electricity that you do not use back to the Grid. If you buy the panels, you own the right to sell the electricity back to the grid. So think of it as an interest-free loan to buy the panels, the repayment of which is enabled through them taking your right away to resell the excess electricity. The contract period is typically 25 years.
As some others have pointed out above, these schemes will become a lot less attractive for the rental companies after the end of November when the price guaranteed by the Government to buy back excess electricity reduces - it is believed that these schemes may be killed dead.
Even if companies continue to market them, be very clear about who is responsible for MAINTENANCE of the panels over 25 years - you or them. Others above have warned of problems controlling the quality of the installation - if you were paying and the installation was poor, you could put pressure on to resolve by denying payment. You are not going to have that opportunity.
Finally, think very carefully about whether anyone would want to buy your house with these things stuck on the roof.
All-in-all, not worth the risk IMHO.
If you really want solar electricity even the reduced payments from December 2011, consider buying the panels outright