I think I would like a treadmill if I was a bit more active, I honestly do not see me running on one anytime soon. Even when I was in highschool and was more fit running still gave me the worst cramps.
Treadmill aren't just for running. You can get a great workout walking on a treadmill with an incline... sort of like a stair machine... or just by walking fast. Treadmills are really great, because you do a long workout, run for 1 minute... keep walking for 10, run for 1... then after a week or 2, maybe do 10 walk, 2 run, 10 walk, run, or whatever. They really are very versatile.
For me, bikes are just boring. If you get one you like, great... but to me, a treadmill allows you to do many different types of exercises and intensities.
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One more thing: BUY the BEST you can afford! You don't need all the stupid bells and whistle, but if it doesn't feel good, and doesn't work right, it's worthless.
I got my treadmill at Sears, and have a 10 year service plan and guarantee on it. Every year, they'll even come out and maintain for me if I want (so far I've only done once every 2 years)
Sure the plans cost a bit, but I think it was worth every penny.