I've just started attending classes at Orange Theory and I love it. It's meant to push you with various intervals of cardio and resistance training. Everyday is a different workout but it follows a general "blueprint" of up to 30 minutes of sprint/ intervals on the treadmill; then to the rower for a certain amount of time alternating with weights/ TRX/ bosu/ bodyweight training. You have a heart rate monitor so can see your rate on the board along with what zone you're working in. The afterburn effect is dependent on the number of minutes spent in the orange and red zones so they say your target is 12-20 minutes in these zones is best although I've ranged from 21-33 depending on the workout. I only attend once weekly because I like to follow my Cathe routines mainly but the frequency depends on what your goals are. IMO, you can't really build much muscle the way the workouts are set up, but if your goal is weight loss/ cardio fitness it's great for the calorie expenditure and EPOC effect. It's definitely taken me to a new level of fitness but again you get out of it what you put into it, and if you're not pushing the speed or weight load then you won't have an intense workout. The gym by me offered a free workout session to try it out so maybe you would have this offer where you are located as well.
Jamie