I think that, on a whole, the XBL messaging system works great. One major flaw with the current system, specifically for the web-based version, is that it no longer includes a timestamp with the message making it impossible to know when it was received (other than the day). This makes it extremely difficult to coordinate with friends online to arrange gaming sessions, events, etc.
I'm not sure where the best place to post this would be (issues with the new forums is an entirely different topic), so I posted it here. I would think that it would be very easy to implement such a feature, especially since it seems to work on the console already. Microsoft, XBL team, please fix this!