I'm well underway using my Mac mini as a home theater PC (HTPC).
Background: After finding myself connecting my MacBook Pro time after time to my 46" Samsung LCD TV to watch videos, I decided to dedicate a computer to the task. I looked hard into buying an Antec case like the FUSION or NSK2480 and building my own PC, but there's a problem. I'm lazy.
So I purchased Apple's cheapest refurb Mac mini a couple of months ago (then $479, currently $679). As always, Apple's refurbs are indistinguishable from new ones. I added a new Logitech MX Air Wireless Air Laser Mouse (I paid $100). I still need to buy a wireless keyboard, but since I use the keyboard rarely, I haven't done so yet.
A Mac mini has some real negatives as an HTPC. The main thing is, it has DVI out, not HDMI. Also, there's no internal expansion, so things like a built-in TV tuner have to be done via FireWire or USB. And the only reasonable way to increase storage is to add an external hard drive like the NewerTech miniStack, a problem I'll have soon enough. Last but not least, like all things Apple, a Mac mini (even a refurb) is generally more expensive than a comparably-equipped BYO HTPC, especially if built via deals on dealnews.
But it's also got some real positives. It's TINY. It's incredibly tiny compared to Antec's HTPC form factor, and yet it still fits in a built-in DVD reader. (That means that I can toss my upconverting DVD player.) It's completely silent, with no noticeable noise whatsoever. It has optical audio output, although I haven't set that up yet.
Also, from a software standpoint, it works really well out of the box with no setup required. Front Row, Apple's media navigator, is free and an extremely simple interface to access my videos and music, so anyone can use it. Just stick your content into the appropriate folders (e.g., "Movies" or "Music") and it's ready. And since it's a Mac, things like setup, file sharing, and so on are all quite easy for Kari or for guests to access. You can use the iTunes Store for video and music (although as I've mentioned before, iTunes is dead to me). It even came with an Apple Remote, which is the only remote you need when using Front Row. (The Logitech Air is needed for things like Hulu.) Lastly, if I really want to use Vista's media center features, I can install it, since this is an Intel-based Mac.
The negatives weren't a big deal to me, although setup without HDMI took longer (two cables instead of one) and I'm not happy with the DVI-to-HDMI conversion (explaining that begets a whole 'nother blog post). As for no TV tuner, I have hi-def cable from Time Warner, so I didn't plan on watching cable TV via this device (even if if there is a CableCARD solution out there). It's solely there to archive all of my DVDs (so they work as video-on-demand), watch Hulu and other online video, and playback MP3s. However, it should be said that no HDMI and no internal expansion make a Mac mini an inviable HTPC for most home theater enthusiasts.
As easy as setup was, it took me a while to get around to setting it up, then it took me a while longer to add content to it. But now that I'm using it, I'm quite happy being able to hit one button on my Onkyo receiver and having my HTPC and all of its content available instantly. Now, I want to start reading up on what other people are doing with their HTPC Macs to see what I'm missing out on.
More to come.