The elliptical machine is one machine I never recommend using or put in anyone's program. I do firmly believe that the best exercise you can do to lose weight is the one you enjoy doing, but unless someone is a die-hard fan of this particular cardio machine, I will never put anyone on it.
The movement this machine mimics is one that is unnatural and not performed by our bodies in real life circumstances. When you use a treadmill or a bike, you will become better at walking/running and biking. The elliptical will not help you run faster, bike for longer, or perform any natural movements with more efficiency.
The main reason people like the elliptical trainers are because they burn calories and are low impact. Well, unfortunately the calories you burn are actually less than what it will tell you on the machine; maybe 50-60% of what's shown is what you actually burned. The same is true for most cardio machines really. If two people of the same weight, height, and sex got on a cardio machine, one of them bikes 5x a week and is made up mostly of muscle, and the other does nothing and is made up mostly of fat, they're not going to expend the same amount of energy. Calories burned will generally depend on muscle mass, skill level, age, and sex.
As for the low impact argument made in favor of the elliptical- well, if your body can't handle impact from something as natural as walking, that needs to be addressed, not ignored. Impact will make your bones stronger and jointes more durable. Your body needs a certain amount of impact and all of these low impact exercises could actually make us weaker!
One of the best types of programs with the most benefits would be a circuit training workout. Join your cardio and strength training together, with little rest between exercises, and you will burn major calories! Combine a squat and a shoulder press; perform a step-up with a bicep curl; add an overhead tricep extention to a lunge. Do jumping jacks or run on the spot between each exercise and you'll be sweating like a pig! You'll be working so many muscle groups at the same time that the calories will burn off quickly and continue burning even after you're done the workout. You'll be more efficient in everyday situations because your body is use to performing as a unit.
If you truly love using the elliptical machine, go ahead and keep going with it, but know that it's super important and beneficial to your health to add some exercises that mimic the stresses your body is under outside of the "gym setting" as well. You'll see amazing results!