When we saw the demo at team summit they said the VOD movies were free, and if you had a fast (over 3mbit) internet connection you could start watching in a minute or so. But, the movies expired. Even though you had them on your DVR they would be auto deleted at the end of the period. So, if Starz had a movie on for the month of August, your copy would be deleted at the end of August. This is of course different from when you record something on Starz using the normal DVR functions.
So, the trade off is immediate availability (with fast internet), but the content expired. You could of course download immediately and then set a record via the guide later on to get a copy to archive.
With PPV of course the copy would only be good for so many hours from when you started to watch it. So, if you downloaded a movie today, but did not start watching it until next week, that is when your timer would start (i.e. 1 day rental or perhaps a week rental).