One of the biggest merit pay schemes in the nation is ongoing in Houston ISD here in Texas. Last year's awarding of checks was a debacle, with seemingly no rhyme or reason to the dollar amounts. In dozens of cases teachers who had been let go were given bonuses for the test scores, while others with extremely high passing rates received nothing.
This year's awards don't seem to be much better. Via the Houston Chronicle's School Zone Blog, one top ten recipient from last year dropped from a $6100 bonus to $420. Another teacher who received nothing last year got $7600 this year, without changing his teaching style. One commenter noted that teachers at a school that was rated "Exemplary" for 2 years in a row received nothing.
Meanwhile, of the 251 principals of Houston ISD schools, 232 got bonuses. The superintendent received a 75K bonus, all 19 Executive Principals (maybe someone can fill me in on what that is) received bonuses and all 4 Regional Superintendents received them.