Another disadvantage (in my experience) is how inaccurate the iPhone is vs the BB or GPSr -- even in wide open, non-covered areas. We use both the iPhone and Blackberry apps. In my use, side-by-side, the Blackberry (9700 and 8900) always beats the iPhone. While the Blackberry shows its accuracy being 15-20m, it will always get me within range of a find, while the iPhone wants to to take me on a scenic tour nowhere near GZ.
However, the iPhone app is better at showing information such as TB inventory/information and instant logging, although I've recently started using the old [http://wap.geocaching.com] on the BB for logging, which is good too. Both apps are good; I just find the iPhone too inaccurate to be worth using -- enjoyably anyway . I often find myself using the BB to find, then logging my finds or TB grabs on the iPhone.