I was a little skeptical at first but the more I read, the better it sounds. I doubt it'll replace anything I already have but it would be a great addition to the home environment for automated backups, password sync, etc.
There's a bunch more info here: http://www.winsupersite.com/reviews/whs_preview.asp
Sounds like its not strictly RAID but will allow for dynamic addition of drives and a single share for each type of media (music, pictures, videos...). I like this quote:
It also supports Windows Media Connect so any UPnP capable device should be able to stream content off it (they mentioned Roku Soundbridge specifically in the interview with the product manager on the Media Center podcast).
Of course it's not coming out for a while so I'm sure things will change a lot by the time its released.