x-newa provides a UI to NEWA web services and that is a relatively easy port but it relies completely on xbmc for playback. If I ever do an extender client using native playback, the NextPVR-based NMT interface is better approach, but I'd want a better target then an atv.
the reason I've always like the atv2 as a client (with the assumption that it is just a matter of time that apps will come along) is Plex for it is excellent and Netflix, Hulu and recently more plugins are the best around...combine that with a X-NEWA type of interface and you have an amazingly low powered and feature rich box.....just my point of view...
One could go for another client but then you'd miss out of the excellent Netflix/HULU apps....