I switched from an iPhone 3G to BlackBerry 8900 a year ago and will never turn back.
The iPhone is a lot of fun - it's got neat little apps that can do basically everything except cook you breakfast. I will definitely be picking up an iPod Touch at some point to play with the apps and to use the better browser.
The question is what your needs are. Do you need a robust, business-oriented smartphone with the ability to run a few basic apps (twitter, facebook, weather, sports scores), or do you need a multimedia, app-based toy with the ability to make phone calls tossed in as an afterthought?
Personally, BlackBerry has almost universal acceptance amongst my peers (professional students/young professionals), so having one means being a part of the club - BBM is an entirely different form of communication. I certainly did not use the iPhone's threaded texting the way I use BBM.
I also much prefer certain aspects of the BB OS to the iPhone OS. I like the way certain apps can be integrated into core OS - for example, you can send a photo to Twitter or Facebook directly from the camera app, rather than having to go into the Twitter or Facebook app. The notifications on the BB are much more robust than the iPhone - the blinking LED is a godsend, and the BB has status-bar notification icons. These help me stay on top of my messages. The BB is also capable of full background processing. I open and log into the MSN Messenger app, then close out to the homescreen or run other apps and continue to be notified of new messages (through any combination of ringtone, vibration, and LED) without an annoying popup in my face. I like the way I can start typing a contact's name from the homescreen and their number will pop up and I can dial it (as opposed to opening the phone app and scrolling through all my contacts).
Anyway, that's my spiel... the iPhone has won the app war, hands down, it's just a matter of whether all those apps are for you. BlackBerry has had years to refine itself as a phone/email/messaging device. That's what's most important to me, and that's why I prefer a BlackBerry.
edit: some caveats to the BlackBerry:
- the music player is functional, but I don't use it as my primary music player the way I did with my iPhone
- Mac syncing is hit and miss - I've been pretty lucky with it, but others haven't. Google Sync might be the best option for this (I'm using it now for calendars, and so far it's been flawless)
- if you don't get a newer model, expect frequent crashes
- some apps require you to restart after installation or uninstallation... depends on their level of interaction with the OS, but still annoying
- two-way GMail sync is supposed to be implemented soon, but until then, it doesn't do it