I have the first and second general Roku boxes. Yes, you can use Plex on them. It's just a channel to install.
To find a discussion on it, you can go to the Plex forums and read the Roku subforum.
As for useful, that's a bit subjective. It's as useful as Netflix if you have a digital media you keep stored somewhere in your home network.
I would suggest checking out Plex
's website. I think you may not understand what Plex is or what it does.