I'm okay with value-added metrics, so long as it is not the only tool used to evaluate and assess.

1) Not all things taught appear on a test, or can be easily measured, and

2) Teachers are not the only entities responsible for student growth. There is also the curriculum, the social environment, the home life, and so on.

3) The natural reaction to such metrics, particularly in the current educational climate, is "Holy crap. If I don't do well on this, I'll get fired." Rather than jumping on the "Let's fire 'em" wagon, I feel that comprehensive professional development plans, based around highlighting strengths and excellent patterns, would accomplish just as much. Instead, though, the current professional development plan seems to be "Fire 'em".