I guess you'll start encoding your stuff in that format? And is it very cpu intensive as divx seems to be?
DIVX CPU Intensive? Do you mean when encoding it or playing it? Playing DIVX files isn't CPU intensive at all, but encoding sure is. As for encoding RMVB files, I couldn't tell you if it's more or less CPU intensive than encoding DIVX because I have such a good computer. I don't notice that sort of thing.
I will currently be converting all of my LBT vids into this format once they come out with a version of VLC that will support this new format and they come out with a plugin to support it in my music video making software. As for now, I'm sticking with DIVX because it is currently more flexible than RMVB files. I hope that will change however.

Is this download compatible with Mac OS X? And is it free of charge?
I don't believe that the alternative to real player that I linked to in compatible with macs, but I know real player is. Yeah, ya have to go through a lot of crap to get it for free and ya have to jump through all their hoops, but you can get it for free. They try to make it look like it's free and then they give you some crap about your trial being up. I wouldn't download it directly from real players website. I'd download it from
download.com to avoid most of the crap Real Player makes ya go through.
EDIT: Not surprising, this new format was invented in Japan. It seems that nothing technological is invented on this side of the world. <_<