I have heard about it, but don't know a whole lot about it. There is not a ton of support yet. From what I have read it sounds cool though. It is supposed to have a better layout for cooling.
Yeah BTX is suppose to eventually replace ATX, it has some nice features, somethings that have been needed for a long time. There was a BTX review at anandtech.com a while ago if you want to read up on it.
