I see everyone's concerns about this, I really do. If applied as a blunt tool in the way some of you are describing then yes, it's not going to be fair.
However, that is not the only way.
At my (primary) school this year, we have been trialling a new procedure for performance management. It is not in any way perfect and is being reviewed after the PM reviews next week.
It revolves around being scored for various things throughout the year - so for every book trawl, lesson observation, learning walk...etc a score is given, which is then turned Into a percentage.
We also go through the teacher standards and score ourselves on them using the 4 Ofsted categories. They give us a percentage too.
We also get scored for things like how many 'extras' we do. How we lead a particular subject, how many extra curricular things we do...etc.
At the end of the year, the scores are given an appropriate weighting (which gives lesson observations less weighting than book and planning scrutinies, for instance) and a final percentage is reached. This is then judged against your career stage expectation (so an NQT would e expected to get 75% say, where a senior leader would need more like 90%) This determines whether you are on track.
There are also negotiated PM targets as before, but these are based on points progress- so you look at the individual class and work out what realistically could be expected of that particular group of children in a specific subject. You can always argue at that point that a certain number of Children are unlikely to make more than x amount of progress because of y and that is taken into account.
The Head argues that this system, due to looking at the 'whole' teacher, gives a fairer picture. It's not solely progress related. It recognises that some teachers are better at some elements than others.
I will come back and tell you whether it has worked next week! It is a bit complicated and I do feel as though I'm spending more time being marked and monitored myself than the children are.