I have a Blackberry Bold on Verizon and an Iphone 3G on AT&T for work. I work in the IT Dept. and support on users on these models. I have a Droid on Verizon for personal use. All have unlimited data plans.
The Blackberry is great for the business user, especially the international traveler as we can easily change them to a global plan. I like the Droid very much, but I have one only because we got a 2 for 1 special when my husband needed a new phone.
My favorite is the the iphone and when it comes time to replace my Droid, I will get one. I also support iPad users at work so it's easy for me to switch between the iphone and the ipad as the operating systems are so similar.
I use all of them to check email. My iphone and ipad have personal programs on them such as checkbook register and password keepers.