These programs have been around for a long time. This isn't the same as going to an accelerated rating course where you get all your experience in a few days. For the A&P or IA prep courses, you must show up fully expeirenced, and all you're getting is some quick prep before taking the written, oral, and practical tests (essentially just the written for the IA, which has no specific oral, and no practical).
A flight certificate or rating can be hammered out quickly because it's very specific hands on training. However, the mechanic certificate is far too broad in scope for someone to arrive and learn what they need in a few days. People who are successful at these courses (most are) show up with the requisite experience and ability to pass the test. They then go through a cram course of study specifically for the test, and take all the tests in-house.
Perhaps the best known company doing this is Kings, now Bakers, in Tennessee.
If you pick up a copy of TAP, you'll find perhaps a dozen or so companies advertising for this same thing.
As for the IA, all that is required to obtain the Inspection Authorization is a written test. Traditionally the applicant visits a FSDO to obtain the authorization. There, he or she presents evidence of the necessary experience and then obtains an authorization to take the knowledge exam. Prior to issuing that authorization, the inspectior reviewing the evidence of experience will usually ask a lot of questions. This is done informally; it's not an oral exam, but the person who doesn't pass won't get the signoff to go take the test.
The prep courses simply do all this in one place, and prepare you for the questioning with specific study questions and answers. They then administer the tests in-house, and you can leave with your certificates and ratings. You must be fully qualified upon arrival, however.
These courses are handy, but not necessary. When I did my initial mechanic certificate (A&P), I went to a FSDO near where I was working, and obtained the authorization to test. I then took the tests on my own. I later arranged to take the oral and practical with a designated examiner at a community college, and passed without difficulty. There is ample study material available to prepare for any of the tests.
The big difference in attending these prep courses is the time involved, and the convenience of leaving the course with the certificate in-hand. The disadvantage is that retention is often directly related to the time and effort involved in preparation for a certificate or rating. When you go to these courses, you're getting just enough prep to specifically address the tests, but no more. The mechanic certificate is the most comprehensive certificate in aviation, far and away beyond any flight certificate. It's really not something one should approach half-heartedly.
In all honesty, I wish I'd gone to a school for the basics. Over many years on the job, I've learned quite a bit, but much of it is material that I would have already known upon leaving the school. For those considering getting the mechanic certificate, I highly recommend enrolling in a dedicated school, for what it's worth.
