The science behind HIIT is that your body will coninue to burn fat up to 48 hours AFTER your workout. So, you really get much more bang for your buck and HIIT is probably the best type of workout to maximize your fitness. It challenges your body consistently so it can't adapt causing it to work hard and burn more calories. High Intense Interval Training activates E.P.O.C. (Excess Post-exercise Oxygen Consumption), another metobolic effect where your body's oxygen consumption remains elevated as the working muscle cells restore physiological and metabolic factors in the cell to pre-exercise levels. This translates into higher and longer after-exercise calorie burn. In laymens terms - you continue to burn calories after your work out by simply doing nothing extra.
There have been several studies that recommend 15 minutes of HIIT can equate to about an hour of running or jogging. The premise of HIIT is to push your efforts and rev up your heart rate to over 84% of your Max Heart rate for short periods of time and then recover. For example, you will sprint for about 30 - 60 seconds (or go as fast you can) and then drop down to a light jog to recover for about 2 minutes. All of the exercises in HIIT are built around this concept. HIIT incorporates running, rowing, TRX straps, body weight, dumbbells, bosu balls, kettle balls, etc. Everything is pretty safe unlike Crossfit where you have work with massive weights and are more prone to injury.
The beauty of the workout is that I get a heartrate monitor, which is synched to large screens that display your metrics, so you know your heart rate inensity level and how many calories you have burned. I think it makes these workouts more efficient because there are plenty of times where I think I am at my max but looking at my heart rate and I have plenty more to go. It really helps in monitoring your progress and you receive a progress report in your email after each session.
Oh and don't worry about not being in shape! These workouts adjust to YOUR physical capabilities. If you've been idle for some time, it's a perfect start because you're measured by your heart rate and you will only do as much as your body allows you to. You may be at 94% of your max heart rate by simply walking on an incline, then that is all you do. Nobody knows who you are on the board nor do they care as they are trying to focus on their own efforts. Everyone EQUALLY struggles no matter how fit they are!
I had the pleasure of interviewing the lead trainers of Orange Theory Fitness who specialize in HIIT along with the Heart Rate monitoring system but they are not the only players in town that have excellent reviews. You can also check out Blast 900 and there are several gyms that offer group fitness such as Lifetime Fitness and LA Fitness.