I spent a year teaching the web development immersive program at General Assembly. I felt skeptical about the program too, but I always wanted to try teaching so I thought it was worth a go.
by the way, I'd recommend it to anyone. It's a great way to take a break from dev work and teach the next generation of developers. It is extremely rewarding to watch people go from next to zero knowledge to being able to build functional apps, and they are always looking for new instructors.
So, does it work?
Admissions - We did have a fairly high admissions bar (as to not waste anyones time or money), but the bar was really about motivation and ability to learn the basics (they have to do some basic html / css). The program is VERY fast and demanding and people need to be able to do nothing but code for 3 months. Their personal life gets put on hold and they live, breath, and eat code.
Completion - Every class I taught had exactly one student drop toward the beginning, but outside of that everyone that started the program made it through (thanks in part to the admissions process).
Outcome - Every student that finished the program and actively tried to find work got a job. Each class (of 20+ people) had a couple people that decided to do something else with their lives, but everyone that decided they wanted a job in development got one.
I can't speak for all "boot camp" programs, but I can say that the program that I taught was very effective and IMO people got a great return on their investment.
These views are my own and don't reflect the views of General Assembly or Google, blah, blah, etc..
(edits to fix typos and some re-organization)