Just downloaded it (the page has been down 3 hours or so, I hope a mirror'll be created).
Very very buggy. But promising in the future !
Buggy because it creates an INBOX folder in the main accounts, as well as the Hotmail Bundle: why an Inbox ? Why can't I move it ? Plus, read mail marks as unread every time I relaunch Mail. Finally, it won't send... so that's half a plugin.
But I repeat, I like the idea a LOT and I'll support it 100%.