I am currently using a trial version of Trilead VMX to backup ESXi 5.1 free version virtual machines. This software is the only solution I have found that is easy to use and works with the free version of ESXi 5.1 (be aware the free version does not include the storage API for use in conjunction with say Backup Exec!!).
The backups process well, quickly and rarely fail BUT... sometimes users report that their Exchange password doesn't work.
Once the password is reset all will go back to normal. VERY STRANGE!
Has anyone else had experience of this type of issue????