I used to be a BlackBerry user and then switched to iPhone. Now I am back to the BB world with a BB 8900.
It all depends on what you want. If you want a phone that can do multiple things at once ie: browse the web, have your weather apps or messenger apps running in the background while you do something else. The BB is the phone for you. Media wise as well the BB is getting better a lot better than in the pervious versions.
If you want to play games on your phone all day long and do not care about multi-tasking you can go for the iPhone.
I do a lot of texting and IM. I hated the fact that the iphone would turn off the wifi is I did not touch the phone for 10 - 15 minutes. Not sure if they still do that. The only way to keep the wifi on is to leave the screen on which means u drain the battery a lot faster. Plus keep in mind the new overheating problems the 3GS is having. Search google and you will have tons of links.
In the end, think of what you need the phone for and then decide. I m not saying the iPhone is a bad product its just not the phone for my needs.