I have yet to see evaluations make an improvement in an organization. Teachers tend to know which other teachers in their building are effective or not. Often, the most ineffective and uncaring teachers (those who give our entire profession a black eye) are also the most adept at gaming the system (they have to put their energy somewhere). There are teachers who do almost nothing whom I have seen get the topmost evaluations. They know each "point" on the evaluation sheet and know how to "technically" fulfill them.
At worst, these teachers are rated alongside caring, dedicated people. On the flip-side, I have seen compassionate, hard-working and dedicated teachers receive below average scores. Far from inspiring them to "do better" this sort of rating system only destroys people's self-esteem and creates an environment of hostility.
Now the state has jumped into the mix. I haven't seen the new evaluation packet, but something like 35% of it is supposed to be linked to student scores. Seriously, they might as well link our evaluations to the socioeconomic status of our students. As a general trend, teachers in "well-off" districts (all districts are currently, severely underfunded) will do better than teachers in struggling districts.
This is an unhelpful, unnecessary, and downright harmful system that does not belong in any professional organization. Feel free to disagree, but I have yet to hear of any benefit to formal evaluations.