A research study conducted by Frontend, Real or Imaginary: The effectiveness of using personas in product design, suggests that using personas lead to designs with superior usability characteristics.
The debate about whether personas work or not has been one of faith versus scepticism; claim versus counter-claim. This study demonstrates the effectiveness of using personas in the product design process and while more research is needed, this is now some objective evidence that using personas does work.
The study is far from the definite work that puts this debate to bed once and for all, but it is good enjoyable fuel for the fire and certainly confirms my own belief that, if nothing else, personas provide teams with a way of remembering key audience needs. Well worth a read.