I have had knee problems in the past so running outdoors or on the treadmill is not an option.  I wanted to get a sufficient cardio workout but wasn't sure what to do.  A trainer at my local gym suggested that I try out some of the elliptical machines benefits and from the first moment I was hooked.  The machine had several different settings depending on whether you had an endurance, cardio or calorie focus and because it was low impact I didn't have any pain in my knees.  I was able to get a great workout and burn even more calories than I usually do running.