Elliptical Cross-Trainers are the fastest growing category of cardio equipment. Loved by physiotherapists as they reduce impact from your cardio workout, making them easy on joints. Look for a machine that provides a total body motion (both your arms and legs moving), which allows you to exercise more muscles in less time, with less perceived exertion, leading to higher calorie expenditure.
The most important feature of a cross trainer is the stride pattern. A carefully designed machine that feels fluid and natural is preferred. Since stride patterns differ from one brand to the next it's important to try out the machine and determine which brand and model "fits" and suits you best. Crosstrainers with a "true" stride length adjustment will suit a wide range of people.