I ran on a treadmill last night after work. Given that it was 20 degrees outside, with 3 inches of snow on the ground and sleeting being outside wasn't an option.
I run probably the majority of my miles on a treadmill. I have one in my building, and also belong to a gym.
I have MANY opinions on treadmill running, and have done a lot of research on the issue. I will probably devote a separate post to treadmill running, but in sum, I run slower on a treadmill than I do outdoors. Some people say to move the incline of a treadmill up to 1.0 or so to simulate outdoors. My doctor and physical therapist both said this is a bad idea. It places funny strain on the knee and quads to run at a slight constant incline. My therapist said it is like putting a lift in your shoes all day, just not a good idea.
I prefer to run in Central Park, one of the best places to run in the world. The scenery, the company, the experience is very motivating and uplifting. But not when its 20 degrees, sleeting and dark.