Have built/been using an HTPC(s) since 2004.
I wouldn't consider media extenders/xbox/360's as an htpc or a great alternative personally since they are very limited in terms of codecs and file types supported.
Eventually though I imagine going with a media streamer (i.e. Popcorn hour of some sort) when they mature a little more for further ones.
An htpc is a hobby, and definitely not practical for most people and can be very time consuming and frustrating when it doesn't work and you simply want to watch a movie.
Oh yeah specs:
[*]*Current HTPC: e8400, BD/HD-DVD Drive, etc etc using Vista Media Center/Media Browser Plug-in, Arcsoft TMT3 for BD playback, HD Homerun for QAM HD recordings.
[*]*Separate Unraid server, 9TB storage (currently 6tb but got 3tb more to install now)
[*]*Onkyo 805 receiver.
[*]*Nothing special speakers (in a small condo right now)
[*]*Sharp 42" 1080p LCD
[*]*Optoma HD70 720p Projector (will upgrade when I buy a bigger place)
[*]*4x2 Component Matrix Switchbox (i.e. Play xbox on the projector and watch tv on the LCD simultaneously

)
[*]*Xbox 360
[*]*Harmony Remote (I'm partial to the 360 edition of it since it has the 4 colored hard buttons which map very well to Windows Media Center)
etc etc