Running in reasonably cold weather helps me stay on track for my upcoming winter 5k & 10k events. Despite the cold, I love observing the ever-changing scenery and the brisk air while running outdoors. The treadmill has its place... its a back up for seriously inclement weather. As long as the ground is dry, I am willing to run in the cold.
Layering in moisture-wicking tops and bottoms definitely helps. Wearing a hat or balakava mask and gloves is also very helpful.