I run both daily. An iPhone 3G for personal use and a Blackberry Curve for business.
Why two handsets? Well, aside from the obvious advantage of being able to turn the work one off(!) the iPhone, for me, isn't quite a business tool just yet. Typing out a lot of emails and working on documents on the fly is much much easier on a BB than the touch screen of the iPhone. That's not to say the iPhone is bad, it's just not as tactile for speedy responses as the full keyboard of the BB IMO.
If it was simply a personal usage choice, it would be the iPhone 11 times out of 10.
the user interface is simply the best there is at the moment, and the handset offers just about everything you could want for everyday use. There are one or two downsides (background apps, bluetooth connectivity, flash sites, etc.) but these are largely being ironed out with the new 3.0 software due next month.
Bottom line, get in to a shop and play with both. If the store won't let you play with a 'live' BB find one that will. Any shop worth it's salts should fire one up for you to have a tinker with.