There's the occasional frame rate drops from poor encoding, the frame rate should be constant, usually around 24fps for film. The audio levels are encoded ridiculously low compared to other services like Netflix.
The CDN itself seems to be over-saturated most of the time resulting in drops in bit rate or occasional interruptions. This is especially true on Sunday nights and doesn't matter how fast your connection is.
The highest quality (when you can get it) is 1080p but it doesn't matter because the bit rate must be no more than 5Mbps, maybe even 4.5Mbps.
There are numerous bugs in the UI, just the other day I selected the Series menu and it kept loading movies below it instead. The UX is horrible, it doesn't track what you've watched and there's no way to view recently watched. You can add an episode to your watch list but not a series so when you go back via the watch list you have to way of navigating to the rest of the episodes.
I also think the price is too high, $15 a month is just a little hard to justify based on the amount of content available. I see it more as a service I would just subscribe to a few months out of the year.
Considering the amount of devices you have to write streaming clients for these days (iOS, Android, Chromecast, Roku, PlayStation, Xbox, SmartTVs, etc, etc.), why not just partner with Netflix and offer it as a Netflix add-on using their device clients and CDN?
Realistically it's worth about $5-6 a month and if one could just add that to their Netflix bill easily then I'm sure they would get a ton of subscribers.