The primary reason to own a BlackBerry is immediate access to e-mail. RIM's 'push' technology brings e-mail straight to the phone as soon as it is received. Normal smartphones use a 'poll' method, which is basically calling up an e-mail server to see if there are new messages. If you receive a message in between check-in times, you have to wait for your phone to call in again.
Other features that are attractive are the BlackBerry's ability to be completely customized, from behavior to appearance, which buttons do what and so on. You can tweak to your heart's desire.
Peace,
Father M.