I use a treadmill 1-2X a week. Currently, with no training group, it has allowed me to push myself harder. In addition, I like the idea of controlling the incline and have used that feature to my advantage. I will admit though that it's not necessarily for everyone.
that's a good point. if you tend to have trouble keeping pace on tempo runs ro cruise intervals when you're alone it may be easier to do them on a TM. plus the added benefit of terrifying all the penguins and 'fitness' runners around you at the gym, as you knock out 5:30 miles
While we're at it, I'll provide this long point, which I believe could be the best post to read on this thread:
My workplace has a treadmill. A couple of winters ago, I was running 6 days a week on them and actually went months without running on anything else. I was only going at 0% or 1% inclines. These workouts were base-type runs with plenty of tempos.
After the winter, on my first outdoor run, within 30 yards I noticed how slow it was, because my hamstring muscles were working OVERTIME to push off and were having trouble handling it, due to a lack of strength. My wind and everything else felt fine, but it took about a week to benefit from my winter training. Even so, I probably lost some "stride power" in the process that took a little time to gain back. In otherwords, for the first time in months, the ground was not spinning down underneath me...I had to actually get traction and 'push it behind me', so to speak.
For a lot of treadmill running over an extended period of time, I would definitely recommend some higher incline runs to be included, or something else to address what I stated.
In fact, since I'm likely one of the very few to have completed this "experiment", I think I'll even make a separate thread, because it seems like a worthwhile fact for any runner to know.