I just backed it.
After years of trying to find the right box, I've gone from an intel atom HTPC, to a fullsize HTPC which later became my gaming rig, then to a wdtv live, wdtv live hub, and finally to a Boxee, I really just want something that is simple devoid of all streaming options, can hold my media (able to take to friends homes), and has fast and beautiful interface.
HTPC was nice, but no one else could figure it out, wdtv live devices were nice but laggy input sucked. Boxee is great because it looked great, menus were fast for the most part, and it could pool all the media across various devices in one media library, and honestly still gets the job done.
I think the Slice plus Chromecast fits all the needs I have. Looking forward to messing around with the little box.