Re: this post on Eduwonk, I thought I'd put in my two-cents about TfA. I have met a few TfA teachers in my time, and they have all been intelligent, hard-working and dedicated teachers. While I don't have my hands on objective longitudinal data like CALDER, my experience has been that TfA first-year teachers are just as good as other non-TfA first-year teachers. The Mathematica study, one of the few studies with random assignment done on ed reforms, confirms this observation. I don't think that a convincing argument can be made that, in general, TfA teachers are doing educational harm to their students.
The argument that TfA teachers are detrimental to their schools or are a bad investment for principals is understandable, given that many TfA teachers leave after 2 years. I would be interested to see the rate at which non-TfA teachers leave the same schools that TfA teachers work at. I would be even more interested to see the attrition rate of teachers that are at least as effective as TfA teachers at those schools. My guess is that good teachers in difficult urban schools tend to leave teaching after a short period of time, perhaps to become administrators or to pursue other careers that are more lucrative and less stressful.
Ultimately, TfA's biggest contributions to the education enterprise are in teacher recuitment and selection. TfA has effectively mobilized advertizing and recruiters to transform urban teaching into a highly desirable, highly competitive career. If urban districts around the country were able to convince cream-of-the-crop college graduates to line up around the block to become teachers like TfA, our schools would provide our children with far superior educations.