I had my first attempt at the treadmill today to give it a test run in anticipation of winter and icy roads. I've done all of my running outside to this point - there are significant differences
1. I sound like a herd of elephants on the treadmill. While I'm sure I'm not a gazelle outside, my stomping was downright scary on the treadmill
2. The treadmill seemed easier on my joints than concrete. Definite plus
3. I put a 'random' program on for the incline. Every time the treadmill would raise it's incline I got slightly nauseous as my perspective changed. And I was terrified it was going to tip over!
4. It's really hard to get my pace. Outside I can slow down when I run up a hill, speed up on the downhills, and change my pace to the music/my mood. While I know I could do that on the treadmill with the speed button, it was very, very difficult to find my mojo. As the incline would change I'd find myself running at the top or the back of the treadmill. Kind of scary that you can't self-regulate by slowing your feet down.
Anyhow, it's a nice option for the winter when I don't feel like going to the indoor track. But it certainly isn't easy - kudos to everyone on couch to 5k running on a treadmill. I imagine it makes you a very consistent, evenly paced runner.