An update for Mail seems promising. I am experiencing many issues with Mail at the moment, varying from disappearing e-mails, empty mailboxes (especially Exchange), rearranged mailboxes, interface resets (like menu buttons which keep changing positions), graphical glitches, connection issues and password rejections. One of my Exchange accounts keeps on changing the internal server addresses to a different subdomain. These issues started to appear after the upgrade, so I think they should not be related to my e-mail accounts, but to the app instead.
Overall, I feel there is still some work to be done with Mail. The usability is not fully desirable, starting with the clunky interface to view attachments by which you have to expand e-mails just to see how many attachments there are, only to find no good option to view single attachments without having to either save them or Quick Looking all of them at the same time (and clicking through the individual attachments, which is cumbersome if you have mails with many attachments and no easily discernible document titles). Pre-Lion Mail was much, much better in this. Also, Mountain Lion has greyed out the e-mail addresses in the header, which makes it harder for me to distinguish them with a quick glance (as they grey colour is just too light for me). Overall, I am not very happy with it, yet there seems no better alternative at the moment.