Facebook Messages will not replace email as a whole, but it does have the potential to help Facebook become a much larger player in messaging. Even before this new release I had several people that I communicated with only on Facebook, and I see more and more of this as we move forward.
The biggest benefit I see with the new Facebook Messages is the history for each conversation. Having a threaded history makes it much easier to keep up with a conversation without having to search through email for things that have been discussed in the past.
I also like the Other folder with the new Facebook Messages to filter out non critical or non important messages.
You can also send and receive images and texts from the interface which I think has a major impact on the younger generations as a lot of them simply use texting in lieu of email.