This is intriguing. And apparently their target is Google Gmail coz they name their project Gmail killer. I might just give Facebook email a try once they launch it, but don't bet on anything just yet. Being long supporter of Gmail, I found Gmail has many feature not even Yahoo mail nor Microsoft Hotmail could offer. Will just to wait and when FB launches the email system on Monday.
[update 14/1/2013]
Just a quick update on this quick post which I happened to bump into. I've already got my facebook mail, but it was not all as hot as it should be. It just provided an alternate way to message your friends. Instead of having to log into Facebook, you could just email to your friends Facebook email account and it would pop up in their Facebook messenger inbox list.
So it was not really an email per se, but sort of an alternative communication method to your Facebook friends. If you don't know their email address, or they didn't switch on this feature, it won't work.
I think the biggest contender to the webmail against Google Gmail would come from
Microsoft newly redesigned Hotmail Outlook Live MSN webmail. Now giving this new look a spin and so far I kinda like it. So Gmail would have to work harder to maintain their webmail market share (although they recently surpass Microsoft Hotmail users). The recent Gmail re-design on the composing function was rather not so user friendly compared to their old one. Since it was an on-going process, I hope they would improve it and make more user friendly rather than just trying to make it look nice but became less functional.