Fantastic in what regard? Can I put my favorite linux distribution on it? Can I even write and release software for it under GPL? Without "applying" to microsoft and waiting for them to "approve" it?
Maybe it's just me but I don't really see why I should buy hardware that is 100% controlled by microsoft when it can't do anything that a general purpose computer couldn't do...
Frankly, it works very well for what it was designed to be. 4-5 years ago it provided an awfully intuitive platform to plug into your TV and stream video, tv shows and music from your home network or increasingly from the Internet. Almost no configuration necessary, rarely has issues, and fixes for those issues are pushed fast and often.
It's a home media PC for people that don't have the gumption or wherewithal to build one, and a much more pleasant interaction than many/most of the other HTPC distros out there. And really, almost no one wants to go through building a home server and HTPC, thus the continued advances in "smart" TVs and game-consoles-as-HTPCs.