Most people who still use a Blackberry live on e-mail. Here is a post by a reporter for ZDNet that explains his woes going from Blackberry to iPhone and back to Blackberry.
Saying goodbye to my iPhone, the data hog
Here is the author's usage that gives you an idea of his perspective.
I get between 200-250 emails on an average day. I receive roughly 20-30 tweets or direct messages a day. I get dozens of Facebook notifications, calendar alarms, and numerous phone calls. Each and every time, my BlackBerry vibrates for a whole second, and the LED notifier flashes. Yet even with this, my battery lasts two days without needing to charge, while an iPhone seemingly lasts for only 12-14 hours without charge, but only if small animals are not sacrificed to the Apple gods at regular intervals.
This isn't a story you often hear. The author closes with his conclusion he wants a Blackberry.
My business and work partner for a month, my iPhone. It was fun, don't get me wrong, but I know where my loyalties lie. And perhaps that's what makes the general consumer market so different. We want, we take, and we rarely focus on what we actually need. I need my BlackBerry, and while I still want an iPhone, I know full well it will take time before it needs me.
Unfortunately for RIM this is a minority.