While it doesn't have the greatest variety of content, there are some interesting documentaries and classic shows, and The Man in the High Castle is an excellent series. Considering I was a Prime member long before Prime Video was launched it is a pretty nice bonus feature. I also like that a lot of the videos can be downloaded, which is nice for long trips.
The only real thing lacking in Prime Video is I wish it had the ability to set up different profiles for different people in my house. That way I could set up a profile for my kids and not have to worry about them getting recommendations based on things I watch. It would also be nice if Prime Video was easier to watch on my TV. But overall I am pretty happy with this, and I am looking forward to Amazon adding more content to it.
October 2018 Update: The amount of content is really improving and I upped my rating to 4 stars. Jack Ryan was a fun series, and I am really enjoying Forever, even though it is the weirdest show I have ever seen (and that is saying something). The third season of Man in the High Castle was also the best yet. I am also pleased that there is now a Roku app. I still wish the app had the ability to set up different user profiles, at least on the Fire Stick.