My server logs say I'm getting a small and steady amount of traffic because of the Netbox Cubit 2 and the Hush Mini-ITX PCs. I thought I'd elaborate...
I've got a Netbox Cubit 2 (which runs, among other things, this site). Since I bought that, Hush Technologies brought out the Mini-ITX machine, which is comparable. I've never actually seen the Hush-ITX, but given that it's innerds are 99% the same as the Cubit, I'm guessing there's not a great deal between them.
The Cubit is a small, tall PC. It uses a laptop power supply, which has no fan, and so runs silent (and is of course external to the actual PC box). The machine itself if made of thick aluminium in the colour of your choice. Be aware though, the pictures on the web site are all of the silver case. If you get a coloured one, the case is coloured, but the screws arn't. There are quite a lot of screws, which become all the more obvious when they contrast with the case. That said, the Cubit is a good looking machine.
The Cubit's got a weird sort of swirly shaped air vent on either side. The web site shows a customised shape, which I actually think is much nicer than the standard one. Netbox could do with either doing options, or otherwise consider the design a bit further. I didn't feel like paying the extra for a custom design, so got the swirly thing. I guess customising it will increase the (already pretty long) delivery lead times.
The Cubit's a really squished internal design. You have to take the back off it to get a PCI card in (I put in a PCMCIA bridge card for a Netgear Wifi card). Inside, things look pretty crammed in. It's not the most servicable design, but hey, it's a home machine, not something to stick in your computer room. Getting a PCI card in is easy though, to the point the rear of the case has a little slot cut in it for the little tab on the end of the PCI card's metal thing to go into.
Once running, the Cubit's pretty nifty. The Via motherboard and CPU are really good. I run Redhat 9 on my machine, which has full support for everything in the case. From playing about, the machine is plenty fast for most applications - if you want a PC to work with, it'll be fine. If you want to play Half Life 2, then look elsewhere. Mine's a web server plus a few other things, and it's absolutely happy with that. I haven't seen any comparisons between this and any other "off the shelf" machine - that could be interesting. The onboard Bios is pretty standard, although doesn't seem to have any console redirection capability. It does allow you to set a delay on the (very easy to press by mistake) power button - almost imperative, I'd say.
My Cubit's got no monitor on it, and it sits on top of my fridge. People that have seen it have said things like "you wouldn't know it was a PC". Of course, there are also the usual "looks good", "very nice" etc etc comments.
I haven't actually seen a Hush Mini-ITX, but it's along the same lines as the Cubit. It's innerds are the same, although it's a completely different case design. It's more the sort of shape you might put on top of your VCR or something. I'm not completely convinced you'd use it quite like that, because with a harddisk spinning, it's not going to be that quiet. I guess you could remove the disk and replace it with a Flash card though (like the Cubit, the power supply is external).
The Hush has the same single PCI slot, which looks a bit more traditional than the Cubit's slot. I'm not completely certain, but I think the Hush can take a second harddisk, which could make this machine more useful for "home server" applications. Looking at the pictures, it's more tradionally made, so probably easier to take to pieces and service than the Cubit. I can't tell without seeing, it, but I expect the Hush case is more precisely engineered than the Cubit, not something that's going to bother most people, I don't suppose.
So, all in all, there's not a great deal between the two machines, except the case design. The price is similar, the spec is similar, the capability is similar. I'd love to know more about the Hush machine though.