Right now it's in beta testing. There was a time a month or so back when they were giving beta testers invites, but it looks like they're done expanding for the moment.
The storage space is great, but what I really like is the thread format of reply emails. The "star" feature is nice for finding important convos, and the search ability kicks ass.
Speaking of hotmail, is there anyway to turn off that damn junk filter? Both accounts of mine, it turned on automatically and now...im stuck with that shit trying to sift through my mail.