Aside from the fact that running 10, 12 or more miles on a treadmill would probably kill me from boredom, I make a point of doing my long weekend runs outdoors. I have to get up extra early in the summer and/or run wherever the shade is, but that run outside is really important to me.
But for shorter runs, of say 6 miles or so, I have no problem hopping on the treadmill to avoid running in an 85 degree afternoon during the week. Such as it was yesterday for my scheduled 6 miler at a 9:22 pace. An easy run in week two of marathon training.
The problem with this kind of run is that while I sweat like a maniac it's a generally too easy workout for me, by probably a minute or so a mile. Especially on a treadmill.
To compensate for the slowish pace (which I'm dealing with because I'm trying to run 26.2 miles and the key is to slow the heck down) I find myself cranking up the incline on the treadmill. While that turns me into a true salination machine, it gets my heart pumping a little harder and challenges me without going at a faster pace than I should.
There's nothing like running outdoors, but there are ways to make running indoors a little more manageable. For me, playing with the incline buttons is key.