..And I have found many benefits to running on a treadmill vs. slogging through slush, ice, wind, and battling traffic.
I use my heartrate monitor. Helps me hit my goal effort for a particular day - be it recovery or a workout. I did a 21 mile run last week on an awful day. 1/2 outside, 1/2 on the treadmill. A great way to break it up and get a quality run in. I risk injury doing that much on sloppy footing and in the cold.
Using a heartrate monitor is best since treadmills are woefully innacurate.