In general, the Zbox's are great pieces of low cost hardware. If you decide to go the Linux route, get one w/ an Nvidia ION chip to save some driver headache. If you decide to go with windows get the Zacate E-350 Zbox for some added CPU horsepower. For a HTPC uses your major limitations w/ these platforms is that Tuners are hard to add if you want to go with MCE. Just any MCE USB tuner will eat your CPU alive w/o a built in MPEG4 Encoder. Gaming will also be a somewhat limited experience if you desire to do that. Finally If you have Netflix, you wont be watching your content in 1080p since Silverlight doesn't have hardware acceleration.
Right now I am running an E-350 system, that I have had for about 9 months. I have really had a great experience with it Running XBMC, Boxee (Before they abandoned the PC version), and Chrome Browser for web video with my IOGear Keyboard+Trackball. Two days ago I just ordered the the new G530 Celeron 2.4ghz Sandy Bridge w/ an ASROCK mATX H61 board. Together they cost about $130 (not much more than a Zacate setup) and use not much more juice than the Celeron. Media Center takes a bit of time to get going on my E-350, and I needed some Card slots to do TV recording.
The E350 is a low powered, low cost solution to HD Video with a few minor limitations that are really not that hard to deal with. I would stay with my Zacate board if I didn't want to Record TV w/ MCE, and satisfy my desires to tinker with things.