Personally, I think treadmills s*ck, unless you have a fan set up for cooling, you can't get a decent cardio workout without overheating - or more accurately, you overheat before you can get a decent cardio workout. Cycling is a good way to get a hard cardio workout.
I've actually considered getting a bike....I have a pretty good 35-40 minute treadmill program, though lately with the baby being born and my crazy work hours,it's been tougher