I live on the second floor and while I like running it'd be nice to not have to go out in the cold sometimes. I'm afraid using a treadmill wouldn't sit well with the downstairs neighbors. Does anyone have experience with this?
Here's a link to some good advice: How to Use a Treadmill on the Second Floor.
I would think 90% of the noise/vibration issue will be based on the construction of the house/apt. Without thick, well insulated floors, you'll need to add foam mats until you reach the level of 'quiet' acceptable to everyone and hopefully not to many mats to take away from the use of the treadmill. Lift weights, unless you drop them, there's little noise and you'll get a better workout.