I am really suspicious of any teacher evaluation system that ties to merit pay. I don't trust administrators and politicians to understand statistics well enough to measure my impact on student learning, as distinct from impacts such as home life, previous teachers, environmental factors, etc.
I would love to be able to earn higher pay for better performance. I have been so frustrated with terrible teachers getting paid much more than me throughout my career, simply because they'd been there longer. But I haven't seen any merit pay system yet that is anything more than a token bonus for some nebulous student performance measures.
Nate Silver did an AMA on reddit the other day, and the top question related to teacher evaluation and merit pay:
"What are your thoughts on data-driven metrics for teacher evaluation? Do you think a system that accurately reflects teacher value could ever be created, or will it always be plagued by perverse incentives (teaching to the test, neglecting certain types of students, etc)?"
He basically said that's too big of a question to answer in an AMA[0], it would take a book. But his first line captures my stance on merit pay:
"There are certainly cases where applying objective measures badly is worse than not applying them at all, and education may well be one of those."
[0] - http://www.reddit.com/r/IAmA/comments/166yeo/iama_blogger_fo...