Cameron himself outlined a couple of the downsides of fixed term parliaments in his speech today. (nextleft.org)
Following this motion, legislation will be brought forward to make provision for fixed term parliaments of five years. (politics.co.uk)
With fixed term Parliaments of 5 years, that means seats would change every Parliament (though it would also mean that the changes were normally quite small). (ukpollingreport.co.uk)