I've been going back and forth between BlackBerry and iPhone since the original iphone launched in 2007. At the time I was using a BlackBerry Pearl and waited nearly a year to get the iPhone. I used the iPhone for three months, sold it and switched back to the Pearl.
After that i went through a few more Blackberrys (a Curve 8310 and then a 8900) until the 3gs came out last summer. I upgraded to it in July and used it for a good four months or so until I began switching the SIM card between that and my BB 8900. When the BlackBerry died, I went back to the iPhone until I received a BlackBerry 9700 as a gift this past January.
The truth is, I just couldn't handle the lack of apps and anything else that could keep me interested in using the phone. I like productivity apps, and that is something I feel BlackBerry and RIM are sorely short on. The iPhone, as we all know, offers tons of options.
Before AT&T messed with the data plans, I switched back to the iPhone. For good. I can deal with whatever shortcomings may come (battery life, etc), and am hoping to upgrade to iPhone 4 just as soon as I can afford it. My boyfriend is taking my BlackBerry 9700 and will be using it for work, and I can't say I'll miss it all that much. I do now, though, that it's going to take something pretty awesome from BlackBerry and RIM to make me even think about buying another of their devices.