I've been actually part of the BETA for over a month and a half now but have no invites etc as yet.

What I like about it?
Ease of use to manage multiple hotmail and web accounts (supports other webmail clients). RSS integration, Blog it feature, Photo Email (ala MSN Premium). Integration with Messenger (display picture/name and PSM).

What I don't like about it?
Performance (early BETA) is sluggish.

What I'd like to see?
It could do with keyboard shortcuts (to mirror Mail Live). Direct blog posting to Spaces without spawning a browser session. Messenger Tabs (I love my stock tab and xbox live tab).