I switched from a BlackBerry BOLD 9000, to a BlackBerry BOLD 9700, then to a 32GB iPhone 3GS. Two weeks after the iPhone 4 came out, I got mine, and well, I'm still here...
I've tried to go back to the BlackBerry (just because I was faithful to it for YEARS, and I manage BES at work), and each time it fails! I still have a BlackBerry BOLD 9700, and a BlackBerry Torch...
Every couple of days, I turn them on, and use BlackBerry Pin Messenger for a while. But, to be honest, that's all I do with it.
So, I guess I can officially say, I'm iPhone all the way.
The best thing I can say is, if you're switching from BlackBerry to iPhone, and you have a GMail\Google Apps account, it will really be a seamless transition; data wise...