I have a Shuttle MiniPC that I was originally planning on using. There are a bunch of problems with it though. First, I wanted to find a way to get it to boot faster. I figured Linux would be my best bet, but I remembered when I started tinkering with it that I don't have a clue in Linux, and my touchscreen doesn't have Linux drivers anyway. Shutdown would also be a problem since I don't want to do it manually and then wait for it to finish before shutting the car off. It'd also use more power than I'd like, BUT.. a laptop solves all the issues.
The batter backup means I could shut the car off, have Windows set to go to hibernate or suspend when it switches to battery power, and then quickly come back to the state I left it at when I come back to it, rather than waiting 5 minutes for it to boot. It'd also be nice to have the larger display when I actually want to do some tuning or logging rather than being limited to the 7" LCD. And it'd be using less power with a 12v adapter
made for it rather than a transformer connected to the battery. I really can't see any reason to use a PC over a laptop other than wanting to show off.
Now, I have read (very little) about ways around the battery backup of a laptop -- "intelligent" powersource that'll kind of do the same thing. My concern is that the PC may use more power even in hibernate and drain my battery, say if I don't start the car for a few days or a week. I also can't see spending the money on these things when I could spend the same amount on just putting a laptop in, but if I could I'd definately go for it, since I already have the MiniPC. I haven't done a lot of research on it because mp3car.com is blocked here at work (where I'm usually pretty busy anyway) and once I get home after 11 hours of work (on a computer all day) and a couple hours of class, I don't feel like using a computer and don't really have the time to anyway. If I'm way off and the MiniPC would work just as well, let me know! That'd be great!