It depends on your goals, and what level of fitness you have today.
It is easier on your joints, and people with limited fitness levels or physical capacities can often push themselves harder on an elliptical than they could walking or running. I was able to work on an elliptical even when I had a hip that was in such bad shape that it needed rebuilt, and started back on an elliptical after my surgery.
On the other hand, the exact things that make it good for some people makes them less suitable for people who are already athletic. They are easier on you, will not push your heart rate as high, and take less muscular strength. If you are really trying to reach your peak athletic performance, elliptical training might not be enough for you.
If you can stand doing your exercise exclusively on some indoors machine such as the elliptical, then sure. Get that heartrate up.
But I'm afraid the population of people who can do that is rather small. Don't set yourself up for failure, do something you enjoy and better yet, join a group.
If you really want to take your elliptical outdoors there's always the ElliptiGo: http://www.elliptigo.com/ I've seen a surprising number of these around Boulder CO.
