Hello Kevin,
If performed correctly, HIIT certainly will help you shed weight while increasing your fitness, largely due to the physical and physiological effect that it has upon the body.
Concerning weight loss, due to the fact that the body is forced into using its anaerobic energy system while performing high intensity intervals, glycogen, stored in the muscles is quickly depleted (hence the reason HIIT intervals can only be performed for a short amount of time). Following depletion of stored glycogen, to continually fuel the intense intervals, your body begins to rely upon other energy reserves (namely fat), drawing excessive amounts into the muscle cells in order to repeatedly keep your legs turning over throughout those 30 second intervals of pain.
The by-product of such an event (utilisation of fat as energy) results in a reduction of body fat. As such, by repeatedly performing HIIT sessions 2-3 times per week, you'll quickly begin to see the results of your efforts.
On a physiological level, HIIT forces numerous changes and adaptations of the body's systems, ranging from increased ATP density of muscles and the heart, allowing you to perform the intervals for longer/increase the level at which you perform those intervals. This is largely due to the fact that HIIT increases mitochondrial activity of the heart as it quickly adapts to using lactate, produced by the muscles, as an energy source, thus increasing its efficiency.
The advantage of increased mitochondrial activity in the heart (obtained through HIIT) means that your heart is better able to fuel itself during aerobic exercise (steady state cardio), as there's less of a demand for oxygen. This also results in lower resting and exercising heart-rates.
Additionally, due to the huge energy deficit that each HIIT session creates, there's a massive rise in one's sensitivity to insulin, with the level remaining raised for up to 72 hours afterwards as the body recovers. During recovery, increased insulin sensitivity means that less needs to be produced by the pancreas, as it breaks down carbohydrate into glucose, allowing it enter the muscle cells, as glycogen, in order to replenish them. Don't for one moment think that you can repeatedly perform HIIT in the absence of carbohydrate from your diet.
Don't worry about the effect of HIIT upon muscle mass, as the HIIT intervals are too short to have any detrimental effect. If anything, due their improved ATP density, you'll no doubt find that your leg muscles actually increase in size. Having performed HIIT for three years, the shape and size of my leg muscles have simply gone from strength to strength, largely due to their improved efficiency.
As for exercise equipment upon which to perform intervals, it's important to choose one that you're comfortable with. For example, when I perform my HIIT exercise, I'll alternate between the use of the Stair Mill and the spinning bike, as I use them regularly whilst finding both are great in allowing me to perform high intensity intervals.
Duration of HIIT sessions shouldn't initially exceed 15 minutes (rising to no more than 30 minutes as you become more accustomed) and your recovery periods should be executed without any resistance or tension, as you're recovering. Moreover, by the end of the session, regardless of its length, you should be left feeling completely exhausted. If you don't, your high intensity intervals haven't been strenuous enough.
As for their respective length, initially begin with a 30s (hard)/60s (recovery) split. As your fitness improves, gradually increase the duration of your hard intervals up to 60s, while reducing your recoveries downwards to 30s, before increasing the level at which you perform the hard intervals. However, as you increase the levels, adopt the same principle I've outlined above.
