went though this at the start of the thread, I've tried popcorn hour and for an off the shelve option its not bad but lack of customisation, way it handles custom fanart / posters and limited addons means I would rather have a full HTPC with something like XBMC, Media Browser or even Boxee on there.
YAMJ + the AEON Skin
http://www.networkedmediatank.com/showthread.php?tid=25534 does fanart, posters, etc. Group by season/series & there are plenty of addons for nzb & torrents. Supports Twonky & Ihome, as well as an Iphone app to control your box. You can also use the box as a NAS or FTP or Samba Server.
You also have the MSP (media service plugin) that gives access to :-
Video : YouTube, Google Video, MetaCafe, VideoCast, DL.TV, Cranky Geeks
Audio : iPodcast, Radiobox, ABC News, Jamendo
Photo : Flickr, Picasa
RSS feed : Yahoo! Weather, Yahoo! Traffic alerts, Yahoo! Stock, Cinecast, Traffic Conditions.
What else.. well theres PVR plugins, LastFM, WebBrowser,Ngrab support for Dreambox Users, Gmail apps, googlemaps, photobucket,
shoutcast, weather, etc.
Most importantly, their silent - which is the reason ive abandoned so many HTPC projects in the past.
Apart from a TV tuner, Ive not seen anything an XMBC can do that a popcorn hour can do... unless it can do spotify.