The benefit I have with the treadmill is that I really know the workout I had. I can tell you how far I ran to the hundreth of a mile, how fast I was going, how many calories I burned, what incline I was at, and how long I ran, to the second. For the very beginning, this was important to me. I was able to adjust pace or incline appropriately if I was too tired, or not working hard enough. Also, 6 weeks ago, there was a bunch of snow and ice on the ground, making running outside pretty difficult, if not impossible. This has been critical to my success. I know lots of people hate the treadmill with a passion. It is boring, but I have my music (DH doesn't want me wearing the MP3 player while running on the street).
I can run in any weather, at any time (I never would have done the run I did the other day at 9pm outside). It helps that the treadmill is in my house, and I don't have to go anywhere to use it (it's actually my brother's. He's leaving it here while he lives in my parent's house playong World of Warcraft
With all that said, I have yet to actually run outside, so my opinions are slanted. I may wait until I'm actually done with the program and have gained a little more confidence before venturing outside. I've only got 3 more weeks of the program, so it isn't too much longer. I just don't want to mess with anything, it's going so smoothly!