I have an iphone 3G, which is an awesome device. And I also have a Curve, which i love as well. I prefer to use the BlackBerry, but both devices are great. One reason i like the Curve a little more is because i'm not really a touchscreen type of person. It's cool and everything and helps when it comes to some functionality, but overall, i prefer non-touch. (especially with typing).
Another thing I prefer about the BlackBerry (or maybe all non-iPhones) is from a development standpoint, i don't feel locked in. I don't like the control Apple has over the SDK, such as the AppStore being the ONLY way to distribute your appllication, no trial, apple having to approve your application, among other things.
Overall, both phones are great, with the iPhone leaning more towards the consumer end, and Blackberry leaning more towards the corporate end.
It will be interesting to see how this plays in the coming months... it appears RIM is trekking on Apple's turf more, than apple getting into RIM's territory, with devices such as Javelin, Thunder, and KickStart which would obviously appeal more towards consumers.