I've been part of the Plex Cloud beta for months now. While it is nice, unless they allow for Amazon Drive (or some other unlimited cloud drive), then it only makes sense for the smallest of collections. As it is today, 1TB (which is pretty much the largest that most people will pay for in terms of cloud storage) just allows for a handful of TV shows and movies and music.
Plus, there's still the legality of keeping ripped TV shows and such on a cloud service. Granted, TV shows are still a bit of a gray area, proving you own a ripped copy of a movie is difficult (at best). Music is even more difficult to prove that you own if you get hit with a DMCA notice.