According to this article in CED Magazine, Comcast's VOD service has had more than seven billion views totalling more than one billion hours since the service was launched in 2003. Monthly, there are more than 275 million views totalling more than 130 million hours, and approximately 40 million movies are being watched on-demand.
Comcast's VOD library consists of more than 10,000 titles available each month, approximately 90 percent of which are free to view. By the end of the year, the company expects to have 6,000 movies in its VOD library, half of which will be in HD. (Given the statistics, they most likely have less than 1,000 movies available today, of which only a relative handful are in VOD.)
It's easy to forget, but cable VOD is most certainly a direct competitor for Apple TV, Vudu, Amazon Unbox on TiVo, and similar services. (DirecTV is readying its own Internet-based VOD service for later this year.) I think that it's far more likely that consumers will use the VOD service that comes from their incumbent video service providers than buy and install a separate set-top box that offers much the same content.