Before the elliptical trainer was created the most highly ranked training or workout machines used to be the cross country machines made most famous by the Nordic Track.
The elliptical trainer was designed to most naturally emulate the movement the leg went through, including it's range of motion, and most importantly providing exercise for the upper body at the same time. Because of the thought given to it's design in these areas it will give it's user a much more intense, a more complete, and a more satisfying workout.
The elliptical trainer will give a complete body workout. It does this by exercising more parts of the body at one time than almost any other workout machine and it does this all without placing stress on joints, especially those in your legs and hips. The exercise if very aerobic as mentioned already much like the exercise you would get doing cross country skiing up a continuous incline. Think stair climber and you'll get the idea.
Tread mill running was and is still fairly popular but can contribute to impact injuries you maybe be trying to get over or avoid in the first place. Because the elliptical trainer is very low impact it is popular as a replacement for the treadmill.
These machines are very popular with women who are trying to loose weight or get more toned. They provide a full body workout, burns calories and is low impact - that's why women choose them over other pieces of exercise equipment. The wide range of motion, varying levels of resistance (which can be varied while using the machine in motion) make it an excellent choice for women with these goals. Other machines like the cross country training machines and the treadmills can't even come close in providing the benefits of ease of use and adaptability to the users direct and in-motion input.
For those wanting to achieve specific calorie burn goals the elliptical machine is the way to go. It is generally accepted by most experts that the more you weigh the more calories you will burn when using an elliptical trainer. Because of this there is no set amount of calories a group of users will use on the same machine. It comes down to a number of different factors. The users weight, the difficulty settings chosen, proper use, and the intensity of the users workout. Resistance levels can be changed while in motion.
The sales of the elliptical trainer has increased as people who would normally jog or walk to loose calories switch to these machines. Most explain the reason for switching is that they get better results for the same time input and that exercise is easier and safer to perform on these machines.