My experience with their streaming video service is the free stuff through Prime. (Mostly Star Trek episodes. 😀)
- They didn't properly deinterlace any of it, so it looks lousy on playback.
Movies: Looked alright. The quality wasn't too good, as my connection seems to go to hell whenever I play streaming video.
The other issue I hit was skipping. About 30 minutes into a Star Trek episode, or maybe about 1 hour into a movie, it would often lose the connection to the server. Trying to seek somewhere else would result in the player giving me a connection error message. The page needed to be refreshed in order to keep watching.