Go to
http://ideas.live.com with your Live ID and grab the binaries after applying.
Good
It is like a revamped Outlook Express, with anti-phishing support and junk mail support.
It allows you to set the folder named for 'Sent Items', 'Deleted Items', 'Junk Mail' and other folder names with IMAP, which was one of my biggest gripes about OE's IMAP support. I always keep my Deleted folder named 'Deleted' and my 'Junk Mail' folder named 'Junk', and my 'Sent Items' folder named 'Sent' to avoid any weird IMAP server issues with %20 aka the [space] character.
It has a nice RSS reader built into it.
You can use Active Directory as a Directory service, real easily.
It seems to handle large e-mail accounts and especially newsgroups accounts a lot better than Outlook Express ever did. I originally suspected it was better memory management under Vista with 'Windows Mail' that caused the speed up, but this is with XP.
It runs very fast.
Bad
It requires Internet Explorer 7 for RSS support, which I suspected due to the new RSS APIs that are exposed/added with the IE 7 DLLs.
It has ads when you read e-mail, much like Gmail, but that is expected.
Ugly
There are some minor cosmetic bugs, like "Windows Live Mail Desktop Beta" not fitting within the dialog box of the "About" menu item.