We used Zip for a few months; we got rid of it since neither of us actually has time to watch movies.
Basically, you set up a list of movies you’d like to see (they have a very decent selection) and they will send you titles based on availability. You also have the option once or twice a month (depending on your plan) to have a certain title expedited. You’re limited to a set number of titles per month, but you can pay more to get more.
After you have proven you can send movies in when you say you have, then you start to receive new titles as soon as the old ones are sent, instead of when they are received by Zip. You also get set up with new features if you do things like review films on their website.
All-in-all, we used it specifically to see older and more obscure films, since newer films took forever to be sent.