The Iphone connects to microsoft exchange exchange better than the BB does. BB was a pain to setup and every so often requires that you pull the battery to get it to re-sync up again. The Iphone on the other hand was about 45 seconds to setup and has worked flawlessly.
When it comes to usability think of this -- work pays for my BB and the plan, I pay for my own Iphone and service. I recently gave up working with the BB, and I AM an IT guy, and just give out my personal cell number to those that need it, and stopped carrying the BB with me.
So, I pay out of my own pocket to use the Iphone, rather than using my free BB, does that answer the question good enough
BTW -- stay far, far away from them touch screen BB, they are beyond useless. It ends up with about a 1 second delay between when you touch, and when something (like typing a single character) actually happens.
Don't even get me started on trying to do something like use SSH on a BB.
That, and the IPhone has "air video" which was the killer app for me -- I can stream my videos stored on my home server straight to the phone over wifi or 3G. (No, not the creator of the software and don't get paid in any way).