The Gigabyte BRIX - GB-BXi3H-4010 works well with FreeBSD 10.0, although the Intel HD 4400 Graphics can only use the vesa driver in Xorg which I think means software rendering only. Using a dual-link minidisplay to DVI adapter I couldn’t get it to drive a monitor at high resolution. Certainly without hardware graphics acceleration it will not be much cop to watch video on.
The only obvious bug I found is that if you try to switch to another TTY from Xorg, you loose your screen, and can’t switch back. Apart from that it works very well headless.
Users of the latest version of Ubuntu will be able to use proprietary drivers from Intel available at https://01.org/linuxgraphics/ which should allow for proper (i.e. accelerated) graphics support, so that is probably the way to go for an open source operating system on the BRIX if it is not going to be used headless.
The main pros are that it seems quick, is small, has low power draw, and is completely silent. For someone planning on using the latest Ubuntu it seems like a perfect platform for non-gaming desktop use.
Arstechnica have a good review: http://arstechnica.com/gadgets/2014/04/review-gigabytes-amd-brix-gives-intels-mini-pc-a-run-for-its-money/