I bought a 'mill about 4 years ago. Even though I live in MN, I don't train on it all the time. I'd rather be outside as much as possible. However, I would say that my running got much more consistent after buying the 'mill. Prior to that it was too easy to take the day off due to cold/hot weather, icy/snowy roads, etc. Instead I'd just hop on the 'mill and run.
One thing about my 'mill is that I don't really trust the accuracy. For instance, when I jump from 7.1 to 7.2 mph I can hear the motor kick in and the pace picks up quite a bit. If I have my HRM on, it'll jump too. Instead, I just try to focus on the intensity I want and the duration.
I've also read that you need to put your 'mill at 2% incline to simulate running on the roads. I usually do this, but I don't know if it helps. I think it's still important to get out on the roads a couple of times a week.
I don't think you'll lose anything by running on a 'mill rather than the roads. Heck, in 2000 Christina Clark, who's from Alaska and did all her training on her 'mill, won the women's US marathon trials, which was run in hot weather too.