I agree Remus, when it's good it works, but...
I've worked with a lot of Teach First recruits, as I work in the kind of school that they're targeted at. Some have been brilliant, and stayed on in teaching, and they tend to rise into management quickly, but I wouldn't say that there is a greater proportion of TF candidates who are brilliant, compared with trainees from other routes. It's win-win for the school and the TF person; the school gets a relatively cheap, fresh, energetic teacher for at least two years, and the TF candidate gets a gold star on their CV and they do get excellent, intensive training.
However, I do think students suffer from constant staff turnover. The worst TF candidates are very naive, and incapable of relating to the types of students they are there to help, both socially and academically. I've heard them talk about students as if they were a completely different breed of human, one which needs "fixing". They treat TF as if it were some kind of extended "charity work", like a World Challenge adventure.
But, as I've said, a handful have been excellent - I'm in two minds about it.