I voted yes in the poll above. However, the real answer is a bit more complex. I personally use only PCs. I have my one main TV which is a HDTV connected to a PC. For all my other rooms I use existing PCs with MC and BeyondTV Link or my laptop. For myself, I'd probably never buy a pre-built PC, preferring to build them myself. I've used my PC with my TV in one form or another for 10 years. Originally it was just for music support with visualizers, but now everything runs through the PC.
However, I would certainly consider one of these devices for work and would strongly recommend one (if built well and really delivers) for many of my friends and family. If it were priced right (under $500, preferably closer to $300), I know of at least 2 friends who would be interested and both sets of parents (mine and my wife's) would be interested.
Considering the price constraints, though, I feel a bit differently than Osho on it. Here are some thoughts about capabilities in no particular order:
BluRay: No one that I'd recommend one of these devices to would be willing to pay more for a BluRay player. If this was included, I would recommend it as a build-to-order option only, as it would help to keep the price down.
GPU (Graphics Power): Video game support with a high-end GPU wouldn't be required either. However, I'd recommend against using Intel integrated graphics because it limits you WAY too much in the way of 3D capabilities (even for applications like Theater View) and then you're stuck supporting it for years to come with your software. Nvidia and AMD both have promising and cheap low-end solutions that are a much better option. The Nvidia and AMD solutions also tend to support a much wider variety of displays and timings (such as the common 24p problem) much more easily than those solutions from Intel.
CPU: Whatever. Probably wouldn't go Atom because of the GPU limitations in currently available chipsets. Anything low-end would be good though, and if you aren't looking to move anytime soon, Nvidia is supposedly working on a chipset with an integrated GPU for netbooks that could be a good option.
TV Tuner: This would be a requirement. I completely agree with Osho here that MC is still quite a ways behind the curve in this regard. It is getting closer, but you'd still need a LOT more work before it'd be ready to go. The test here needs to be: the user plugs the cable TV coax in to the back of the device, types their Zip code into the setup wizard, chooses their cable provider (and tier if needed), and voila they have a TiVO-replicated DVR through a Theater View UI with at least as much capabilities as Windows Vista Media Center edition. It doesn't need to be quite as advanced or fancy as BeyondTV or SageTV (at least at first) to satisfy the people I think would be interested, but it does need to be that simple for people with regular analog cable TV. It does also need to be capable of OTA HDTV reception with an antenna, with a similar "type your zipcode, choose antenna, receive guide data for OTA transmissions). Some of the people I'd recommend the device to would be people who are using Antennas for programming at home and will need to buy converter boxes or new TVs for the transition in February.
It would also need a way to work with external cable boxes for people with Digital Cable and satellite service (USB-UIRT external device control). The setup for this also needs to be VERY simple (a standard setup wizard where you "teach" the USB-UIRT the remote control codes using your cable box's remote, and again type in your zip code). The USB-UIRT can be sold separately. To support external HDTV from satellite I would recommend selling and supporting the Hauppauge HD-PVR device. Perhaps two tiers of devices: (1) $300-$500 SD only supporting standard analog cable and OTA reception, and (2) $1000-1100 HD box with a HD-PVR, USB-UIRT, and perhaps even BluRay.
Hard Drive: 250-350 GB would probably cut it. If you go the two-tier route, the HDTV version would require higher storage as well. Whatever you can squeeze into the price point and still keep a nice profit margin. Perhaps consider 2.5" drives for silence, energy efficiency, and size. If that drives up costs too much though I wouldn't worry about it, and it might limit you for a HDTV tiered box too.
Remote Control: Needs something good, RF, and simple. You might have better ideas than me in this regard, but it does need to "Just Work". RF Keyboard and Mouse would be good too so you can use it from the couch.
Handheld Support: You might not like this, I know... However, you really need some hook that will provide value over a TiVo or cable-company provided DVR as a selling point and since MC has such prowess for music playback... I really think the best opportunity for this (combined with the DVR support) is to support automatic compression and conversion for handheld playback. The question people at my lab ask the most is how to get the shows off of their DVR/TiVO and onto their iPod. Automatic conversion to H264 MP4 and uploading to iPods, plus Podcasting support, plus MC's already-existing music/photo handling prowess... If you can make this easy, you've really got something.
All in all, it could be a fantastic device. Theater View is pretty good now as is (especially if we get a few more built-in default views). The main place that needs work is the TV support ease-of-use. You've been working hard on this but it it still feels like it has a long way to go.