Treadmill running

As we head into the winter months, it can be hard to train outside in the sub zero temps but it can also be dangerous with the icy footing. Many people opt for the treadmill for a number of reasons. It can be a topic of mixed emotions for many runners. I know I personally felt shamed by runners in the past for utilizing the treadmill despite exclusively training on a treadmill for a few marathons in my earlier years as a runner with great success. 

Why is the treadmill so controversial?

Why do some people crush it on the treadmill?

Why does the treadmill feel so hard to some people?

Should you utilize the incline feature?