PLEX is nothing more than a Media Server that grabs metadata from the likes of TheTVDB, and IMDB, and gives you an interface to access all the content... There are plugins that let you use Youtube and such inside PLEX, and it also has its own DVR functionality, but the DVR portion is a paid feature right now. I have PLEX on my server with several TB worth of video and music on it... I told PLEX where Movies are, TV shows, and the Music is... It mostly did all the other work, grabbing posters, episode information, so on. I then on any computer in the house visit either the Web URL it gives me, or I use a PLEX App (Apps tend to be paid for as well...) and watch all my content via that...
It's very useful if you have several people in a single home and they all might wanna watch something... Single location, Netflix like interface, and you can customize a lot of it.
The only limit in regards to ISP... Never try to download 600 movies, and a few thousand tv episodes worth of information from TheTVDB and IMDB... Might not seem like a lot of text, but those pictures add up fast, as well as all the metadata.