What you want to do with it...
Stream over the network? Yes - you just have to side load Plex.
Stream locally? No - but neither can the Roku LT, or the unreleased Roku 1 and Roku 2 pucks.
Stream from your phone? No. Or at least not yet. Whilst the Roku app picks up the Now TV box, a puck must be paired to your Roku account for the app to be able to interact with it.
Hacks? As the Roku pucks are built on Linux, the open source aspects do get released every now and then. However, I'm not aware of any custom firmware or OS for any of the Roku boxes. The best you could hope for is an Android port, but the spec of the Roku LT/Now TV box isn't far off that of the RasPi - and Android ports for that have all but stalled as they don't run very smoothly. As phawk754 says, most of the extra stuff you would want can be side loaded.
Netflix? Not via the channel store. Though I don't use Plex, I've recently heard there are Netflix and Hulu plugins for it - which might be worth investigating.