During the Iowa winters, I often am sequestered indoors for a majority of my training. Many people prefer training for the bike leg indoors, which I certainly understand. It is much safer for one, and you can be much more efficient with your training. Repeats and intervals are much easier and you can get a constant resistance for FTP testing, etc. I also don't mind the longer rides being I can catch up on movies/tv shows that I've been missing out on. I typically plan my long rides during the winter around the Vikings games. It is interesting to see the heart rate spikes when AP breaks a long run (or more commonly when the defense lets up a big play).
However, I hear a lot of people complaining about the treadmill (or Dreadmill as it is often referred to as). Personally, I don't mind it. In previous years, I dreaded the treadmill time but typically moved there when there is snow, ice, or high wind chills. Now, during the winter I tend to do all of my run training indoors until the temperatures rise and stay there. There are really no hills around me, so it is a great place to be able to do hill repeats. I have also found it helps me push a bit harder on fartlek and longer repeats as well. Finally, most of the races I do are in hotter and humid environments so it helps me get used to the amount of fluids I will use during the races. Certainly there are downfalls as well. Waiting in line is one. Another is the distractions from people talking to you during your workout. Also, the fact that you are staring at the same spot on the wall for the entire workout is another thing I struggle with. I typically cannot read the captions on the television when I run on the treadmill, so I stick with the headphones and Ipod. I have found that going through audio books while running passes the time really quickly.
Despite the drawbacks, I'll keep using the treadmill as an important training tool. It typically doesn't take me any time to hit my desired paces when I make the transition back outdoors. I am looking forward to getting back outdoors, but with the 20 inches of snow we are supposed to get tomorrow that may not happen for a while. Until then, I will continue to try to make gains indoors!