Just read this rather interesting article about how Microsoft is abandoning their rating system which has been around for quite many years.
Although quite many MS employees probably like the fact that this is going away I also believe the system had some good traits.
One aspect is the difficulty boxing people in, but the calibration part of the system which the article doesn't really mention was in my opinion part of ensuring that people were treated fairly. The fact that different managers had to defend their ratings towards their fellow managers was a good thing. It was not easy to favour a bad-performing individual just because you liked him/her - and the ones truly shining were also identified in agreement with other managers.
It will be interesting to hear how the new system will deal with the cross-team calibration aspect in the future.