If you are looking at getting your dog a treadmill for exercise in addition to ample time spent outdoors, then go for it!
Canine treadmills is strictly limited to their role in physical rehabilitation. In that setting they can certainly be beneficial.
Treadmills also have a role to play in helping some overweight dogs lose weight.
Treadmills are no substitute for outdoor exercise. When a dog goes for a walk or run, chases a ball in the park, etc., the activity engages his mind and all of his senses. He sees and hears new things around every corner, smells an entire universe that we cannot even begin to comprehend, might get a chance to "chat" with a canine friend, and so forth. The mental stimulation that comes from getting away from the familiarity of home and into a new environment is at least as important as the exercise.