I have feet trouble, so if I can't bike, I use the elliptical. I get a MUCH better cardio workout on an elliptical than I can get from walking by doing intervals and/or cranking up the resistance. If I try to power walk (even on a treadmill with an incline), I get shin splints and the foot starts to act up.

So for me personally, I'll take the elliptical over walking any day. Of course, I would also much rather be outside...but when that's not an option, the elliptical is a good solution for me.