I have Comcast on Demand , and while it's true that they have a decent list of movies available, it's not very comprehensive, doesn't have the foreign film selection I want, and you can't go back and get copies of your favorite cable series unless you subscribe to one of the premium cable channels.
I'm a little wary of the download times. They say that with a broadband connection of 5Mbps or greater, that a movie will take an hour to download, and a one hour TV show will take about 30 minutes. That's about how long it takes me to download TV episodes from iTunes now, and if you're willing to schedule it for later in the evening, or before you go to bed, it's not a problem.
We shall see.
Movies by mail - those days are numbered. Soon, going to the "video store" will have the same, quaint conotation that the local soda fountain did in the fifties.