Still preparing for the change to Windows 7 and the next thing on our list was an Email Client to replace Outlook Express.
Thunderbird wins out over Windows Mail in that it looks and behaves like OE and I'm used to that.
I gave Windows Mail a brief look and it's pretty enough and would do the job for most people. Microsoft's software is so eager to sign up you up for other things. It has built in links for MSN etc which I don't think you can remove. It also went to MS on each run though that might have been an option you could uncheck. I didn't see it though.