Shading is a big issue with PV panels, even a small part of them being shaded can cut down the power dramatically. They are made up of hundreds of little cells all wired in series, the ones in shade have a high resistance so it affects all the others. They put diode by passes on groups of cells to reduce the effect, put they can't do it on a single cells because the voltage of a single cell isn't enough to 'open' the diode. One bird sitting on one of one of say 10 panels can cut down the power generated by half.
There is much up in arms about 'rent a roof' schemes (somebody else buys the panels and takes the FIT payments whilst giving the householder the 'free' electric) its a 25 year contract with 3rd parties that really screws up the legal process of selling a house. Some estate agents are refusing to deal with properties fitted with PV panels because of this (irrespective of whose panels they are). There are also doubts about them improving the value of properties because some buyers don't like the look of them despite the payback they'd be getting if they owned them. They do change the appearance of the property somewhat.
![Image](http://www.pluggy.me.uk/photos/smallpv2.jpg)