This review won't be very long. Essentially, HBO Go is awesome!
HBO Go is an online archive of every HBO series created, with a sprinkling of first run movies thrown into the mix. While flipping through the library, I'm constantly surprised at the number of quality shows produced by HBO, and having the entire back-catalog means you can start a series from the very beginning if you're late to the game on a particular show.
The service is currently tied to traditional cable/satellite providers, but it is a "free" add-on to accompany the normal HBO subscription. Unfortunately, there is not an "ala-carte" option yet for those who are trying to "cut the cable" and live entirely on an internet stream television diet only. This means, that the service is also currently tied to mobile devices like phones and iPads as well as laptops. However, there are plenty of rumors swirling around that HBO is in talks with the game console makers to develop an HBO Go application for them as well -- the question still remains whether or not it will still require a cable/satellite monthly service.
As it is, I'm currently making my way through Curb Your Enthusiasm on the iPad (starting from Season 1). I'd be more than happy to put my DirecTV account on hold and redirect my entertainment dollars directly to Netflix and HBO Go.