PLEX is based on XBMC which is itself derived from a Program called Xbox Media Player originally a product of Microsoft. Where open source development started on XBMC in 2003, Plex, which was initially just a hobby, developed in 2010 into a more ‘respectable’ Commercial product.
If your Media Library is mostly Movies and TV shows Plex may be the ticket for you, As I suggested in the XBMC article it isn’t a good pick if you have a large library of serial videos like Trainings or a large collection of home videos.
One of the main reasons this will be a big success in areas where there are ultra high speed Internet is how it allows families and friend ( neighbors to share their video library).
Where you may have poured over all those videos you pulled out of the 5 dollar bin at Walmart it may turn out that many of these are of interest to your ‘neighborhood’ network.