I have an iPhone, and I know several other people who have Droids. Generally, both love what they have.
One noteworthy difference--if you're wanting to use a Droid to access work e-mail/calendar/etc. vie MS Exchange, depending on your employer's security requirements, you may need to get an app to do that. (One of the more common ones is "Touchdown".) The iPhone has those security features built in to it's native email application. Touchdown requires an additional fee (I believe it's an annual fee), and once you set it up, you need to start that software to access your work e-mail, then go to the phone's native email application to access most personal e-mail servers (Google, Yahoo, etc.). On the iPhone, you can access them all from the native e-mail app.
Also, if you're looking for accessories, the market currently has far more options for the iPhone--although with the growing popularity of Droid phones, particularly those from HTC, I'm sure the options there will grow exponentially in the near future.