Biking, whether stationary or outside, is one of the best forms of exercise, both for your heart and for your joints. You get a heart thumping cardio workout, it strengthens your quads (and your hamstrings if you have cycle shoes!), AND you save your joints from the repetitive impact of jogging and running.
Tip: Position the seat so that your knee is bent about 5-10 degrees to being straight at the bottom of the stroke. Why is this important? You place much less stress on the front of your knee (the patellofemoral joint) when the quads contract on a semi extended knee than a knee that is bent to at 90 degrees. Make sure you are not so high that the knee is hyperextending though! This can cause injuries.
In the long run (long bike?), both knees and hips will thank you for your cycling habit!