I currently have a Samsung Droid Charge. It is by far the best smartphone that I have had to date. My previous devices included a Samsung Omnia which was Windows Mobile driven and I have also used the iPhone in the past.
The Droid in my honest opinion, is the most customizable of the bunch. Very reliable, great battery life and tons of apps that help me with my work everyday. The iPhone just didn't have the apps to support my organizations internal functions. The Android did.
I am not a huge fan of BlackBerry as their email functionality requires a secondary data package and backend server for Microsoft Exchange support. Android, iPhone and Windows Phones support this natively.
I have a Hub that outlines some of the differences, pros and cons for these.