My eternal worry with Outlook is that it puts (or put, I dunno) everything into one ginormous .pst file. I've seen them over 500mb in size.
One glitch, and all may be lost. It does backups if told to, but...
I recently migrated a friend from Outlook to Thunderbird, and she was amazed at the dfference. She didn't use the Outlook calendar, which would otherwise have been a major reason not to change.