I'll throw my $.02 in here. As I've said in other posts, I'm a
distance runner. I in fact have a 15k race this Sunday. I have been training for a while now for it, and can't wait to do it, but I've messed up the inside of my right leg. I have no idea how. It's all swollen and hurts to walk. I might do the race anyway, against doctors orders. But thats another story.
The treadmill will be just fine for running. I personally hate
treadmill running, but I have a short attention span and I just can't look at the same wall for hours on end. If it was a 20 minute run or something, I could do it, but I put many many hours out on the roads a week. I just couldn't look at my wall for that long. I like seeing the animals, the people, the cars, etc. It will get your heart rate up, and it will be great for cardio. Treadmills are actually a soft surface for running, so it's much better on the legs than cement. I'm not saying running hurts your joints, like lots of people like to claim. The truth of the matter is, the softer the surface, the easier it is on the legs and feet. Concrete is horrible. A dirt trail is good. A rubber track is better. Treadmill is one of the best. Some of the worlds best runners use the treadmill, especially during the winter. If an elite runner who can hold a 4 minute per mile pace can run on a treadmill, I'm sure it's fine for you. As for is it easier on a treadmill? I think it is. You don't fight the enviornments, you don't have constantly changing terrain, and you have the treadmill itself offering some assitance. Is it enough to make a difference? No. If you are looking for cardio, the treadmill is just perfect. You will burn the same amount of calories on the mill as the road. If you were trying to become a great runner, I would say you need to run on the roads most of the time.
This post got way too long. Basically, to sum up:
Pros - burns same calories as road running (assuming you don't run up and down big hills the whole time you run on the roads), softer running surface (doesn't hurt the legs, feet, and joints as much),
Cons - expensive, boring as hell for runs over an hour in length
Hope this helps.
-Matt