I have to say that I do not know how useful a threadmill is to have at home.
Personally I prefer to get outside for a run. I guess it depends on where you live. I usually run on the beach in the sand. I think generally most people spend too much time indoors (myself included)
I certainly have used threadmills (in the gym) if you look at the cost of a threadmill, it is probably better to have gym membership, as this will provide other benefits as well; meeting people, weights, classes etc.