For business use that requires lots of replying to email, management of large volumes of email, copy/paste functionality, and the possibility of integration with existing Enterprise systems, the Blackberry is a must.
If you mostly just need to read email and only occasionally reply, you have a moderate volume of email (< 30 per day), and you are more interested in a device you can have fun with on the train and use as a replacement for your iPod, then iPhone is perfect. The apps available on the iPhone are truly killer. Real, playable games that you can fully enjoy. Also, web browsing on the iPhone is beautiful -- sometimes I find myself using it in front of my laptop!
I've convinced both my boss and my manager NOT to get an iPhone and to instead upgrade their phones to either the BB Curve or Bold. They both thanked me afterward when they realized just how much they relied on easy email reply and the ability to "mark as read" multiple messages (come on iPhone!). The iPhone is definitely a more media-centric device, whereas the Blackberry has been refined for the specific needs of business users.
I love Blackberries keyboard for sms/email and ipod for productivity apps (omnifocus)/games/music.
Also, being able to listen to music (with a standard 3.5mm headphone jack, btw) and not worry that I'm missing calls is invaluable (the call just interrupts the music).
//edit: I didn't mean this in a negative way. If a blackberry does what you need it to do in relation to whatever your needs are, then thats what you should be using. While hype is fun to get lost in, "Because its popular" is a poor reason to force yourself to do something in a manner that isn't beneficial.
edit: Just to expand -- I got given a BlackBerry for work purposes and it drove me crazy. Generally terrible user experience, unintuitive, and aesthetically awful. Funnily enough, since the iPhone was my first smartphone, I actually found it difficult transitioning to a physical keyboard. I got too used to be able to button mash and have it auto-correct.
But then maybe they just hate me because I'm still rocking WinMo.
Then I meet my best friend for lunch to weeks in a row. Both days, he's having iPhone problems and just can't bring himself to return it...