First off, may thanks to Plex employees as I think the app has gotten really good. It's now our primary way of watching content.
My main pain point at the moment is the lack of a smooth rewind/fast forward. Personally I'd love to see the previous way it used to work with the preview window, but could still work with the current system if it wasn't for the buffering on each skip.
The only other issue I have is an occasional buffer issue when watching direct play but at least that's listed on the current "known issues" sticky post.