In the current climate of information overload, it's all the more important to become selective in whose advice you listen; and whom you seek out for counsel.
The "personal training" industry, in particular, has become one of the biggest breeding grounds for charlatans, shysters and snake oil salesmen. And while I truly stand by the attitude of "caveat emptor", or "let the buyer beware", I'm here to offer a few guidelines that should help.
First off, understand what you're hiring a personal trainer for. They shouldn't simply be one-on-one bootcamp instructors. They should be there far more as a coach - making sure that your form and technique are everything they need to be, ensuring that the day's program is modified/adjusted in whatever form is necessary to help you move forward, and above all else to teach. Someone who simply barks commands and helps you "get a burn" and fire up some calories is a waste of money.
Secondly - understand that the more people who are training with you, the less the hour is going to be about you and quality will take a hit because of it. So - one-on-one is best, semi-private is next, small group classes and finally large classes in order of efficacy. Bootcamps? Pretty much useless for anything other than burning calories and hurting yourself, unless you're a fitness professional yourself and know how to modify/adapt exercises to your ability. But hey - the physiotherapists love them, because they keep them in business.
Thirdly - if you're working out with a DVD, then all of the issues of the "bootcamp" apply, only now compound them by the fact that there is nobody with any degree of knowledge to help ensure you're doing it right. Plus, most of the programs are, if not lacking any true usefulness, in some cases dangerous and designed by people who actually have no degree or certification in the field.
Finally - make sure your training coach has the necessary pre-requisites to do their job. Some real-world examples: Jillian Michaels, at best, has a 17 year old accreditation that "doesn't expire". I don't know a single reputable certification that never expires, without passing relatively stringent recertification qualifications. Besides, if you've ever seen her kettlebell DVD, you know that many times she hasn't got a clue what she's talking about - I hope nobody seriously hurts themselves trying to follow it. Tracey Anderson, and her "fitness" program - Tracey lacks education in kinesiology, physiology and biomechanics. Tony Horton (P90X)? He's a stand up comic.
As I always say - you get what you pay for. But you're better off hiring a trainer 1x a month (a REAL trainer or coach), getting a program and receiving high level education on the program for the same cost as a month of "boot camps". You'll get far more out of it, and your odds of successful and safe completion just go up exponentially.
And when you are hiring that trainer - make sure they're certified. Ask them what governing body they've earned the certification through - some of the most recognized are the American Council for Exercise (ACE), the National Academy of Sports Medicine (NASM), the National Strength and Conditioning Association (NSCA), and the Canadian Society for Exercise Physiology (CSEP).