It is fundamentally locked into itself. You need to either have a device which can run the web client, or a Plex app for the platform. And that works fine if you're doing this just for yourself (I've run a Plex server in the past).
But sharing media is painful (or at least, more painful than simply sending a link to someone), and the overhead of compatibility was always annoying. Additionally, if you don't like the way the Plex app works on your device, tough, that's the only UI there is. However, there are a tremendous number of apps and applications with UIs galore that all support HTTP streaming.
Now, the last time I fully investigated Plex was about a year ago, but since then I've used friends instances and it always seemed lackluster, or at least not as straightforward as bare-bones HTTP streaming.