Treadmills are very popular among their owners. Walking is a natural movement, and you don't feel much specific muscle fatigue.
Besides your heart muscle, you are also working your thighs, calves, buttocks, and hamstrings.
What Could Be Bad
Walking or running on a treadmill is considered weight-bearing exercise. This means that your legs are supporting your weight, and your joints are being stressed. This is fine if your legs and joints are in good shape. But if you have problems with balance, or have diabetic nerve disease (neuropathy), foot ulcers, arthritis, or if you suffer pain in your calves when you walk short distances, a non-weight-bearing exercise, such as working out on a rowing machine, may be better for you.
What To Look For
Most treadmills have a speed range from very slow (1/2 MPH) to a running speed (greater than 5 MPH), to accommodate a variety of workout styles and different family members. Some treadmills are not motorized, but I don't recommend those. They sometimes require more effort than is safe, and it's difficult to keep your pace steady.
A good feature of many treadmills is variable elevation, which allows you to raise the front end. This simulates walking or running up a hill. This is especially good if you plan to walk, not run, on your treadmill. With variable elevation, you can increase the work load by increasing the angle at which you walk. When you walk up an angle, you work your buttocks and hamstrings particularly hard. push button, spring-assisted, or completely manual. The push button version adds a lot to the cost. For most people, the spring-assisted type is adequate, even though you need to get off the machine to make changes.
If you plan to run on your treadmill, get one with good suspension to absorb the shock of landings, and a long enough belt (at least 50 in.) to get a full stride. Some have an open-front design, which allows for a shorter belt while still allowing a full stride.
Safety features should include an emergency quick-stop switch, handrails, and foot platforms that are wide enough for you to stand on when starting the belt. Some treadmills have slightly raised platforms so that you can't burn the rubber of your shoes if your shoes happen to slip over on the belt area.
Other qualities to look for are a 1 1/2 horsepower (continuous duty) motor, a two-ply belt, a wooden deck under the belt, and a control panel you can easily understand and operate.
Stationary bikes are probably the most common home equipment purchased and often the least used. They are known for their uncomfortable seats and the quick burning pain they produce in your thighs. Yet they are still a very good choice when purchased carefully and used correctly.
Indoor biking is extremely safe--no cars, curbs, dogs, smog, potholes, or risk of falling. Stationary bikes force you to maintain your pedaling, unlike a regular .... bike...