I never could stand the idea of running on a treadmill, mostly for the reasons already stated. A machine simply cannot give you the same experience as running outside.
However, if you really can't find a safe place to run, you might have no choice but to use the inferior treadmill. Crimes happen, yes, but from what I've heard thus far I can't really consider it common.
I think both are fine just as long as you keep running. I've heard that some treadmills have a low impact feel for your knees and pavement is bad. But dirt roads and trails are good for you. But I think it's better to run healthily and keep running. Which means that if you can't run outside the run inside. And if there's no indoor track then run on a treadmill. My personal favorite is to run in the dirt trails of parks. It's so much fun ^_^. I feel like a little kid with like the biggest backyard. With that said, I don't run for competition or races... Yet xD. Maybe marathons someday.
Yeah, I'd have to agree that a treadmill is very boring to run on, but they also are supposed to be bad for your knees.. I'm a cross country runner so i can tell you that running outdoors is much more entertaining and is a lot more refreshing, Just make sure you're running on a good surface but not a sidewalk or a road. That can also mess up your knees.