Boxee looks very good to me. But without some kind of support for capture devices it won't make it into my livingroom. I use a htpc (mythTV/mythBuntu) mainly for watching TV and secundaire for the other neat media tricks it can. Given Boxee brings so much more, but no tuner capabilities.
Why not adding a defaul/official plugin for linux based pc's? this will not cost to much time because the opensource code is there. Only need to think about a generic interface en develop a plugin layer for the default backend (others will follow and based upon this). Media information seem to be already there
ps i know there are some unsupported plugins, but come on... i wont invest my time in it knowing it can discontinue any moment. I rather stay with a stable setup. When Boxee has somekind of tuner functionality or activily supports a plugin to add a backend to do this, i'll reconsider. I keep waching this project and hope for coming features.
Kaffeine (contains most functions but just does not have the looks of boxee/xbmc. I don't thinks the project aims to be a full blown mediacenter, so this could make the perfect partner also).
had MythTV as a suggestion, but the channelzapping speed is currently a to slow and seems not to get fixed in the near future. I would recommend other alternatives.