I've been running the "dogfood" version of Windows Live Mail Desktop (WLMD) for a while now, and found it to be really stable and usable. It's basically a superset of the built-in Windows Mail application from Windows Vista, which supercedes Outlook Express.
WLMD is now available for beta testing (on Windows XP as well as Vista) from http://ideas.live.com and it works against MSN/Hotmail (including the mail from Office Live, so if you sign up for your own free domain name you can pick up the mail without being in a browser), POP/IMAP accounts and other providers' mail services, such as Yahoo!, AOL and GMail. It seems it's been available for some time, in fact 🙂
I was prompted about this when Steve Clayton was being interviewed today on TalkSport Radio, and a caller had asked why Vista no longer gave him access to Hotmail... I guess he was meaning that since Outlook Express isn't the box any more, he was trying to use the supplied Windows Mail program, which doesn't offer the ability to connect to Hotmail... so the solution is to either stay with browser-based mail or to use WLMD.