HBO has done what I hope a lot of others follow in with. If you currently have HBO on your cable or satellite, you can have HBO on the go with you. See, HBOGO, it all comes together.
When you first download the free app, it asks you to set up an account. By logging in to your cable or satellite company, it will be checking to see if you currently have HBO in your package. If so, you're all set. I only played around with it a little bit but it has a pretty big library of movies that you can stream to watch. As well as all of HBO tv shows, and specials.
It doesn't just have the current hits either. It has old specials and TV shows as well. Like Band Of Brothers, Deadwood, and Carnivale! And it even has some of the "late night" movies. So you can take some softcore porn on the go! WOOT! And as for its movies, it has everything from Avatar, to Teenage Mutant Ninja Turtles.
On nice thing is the watchlist feature. If a series is currently on TV, whenever it is ready for viewing it is added to your watchlist. It is almost like having a DVR on your iphone, ipad, or android smartphone. Almost. In a small tiny way.