I'm a relatively inexperienced programmer (< 2 years in Java), with absolutely no prior experience with database work at all. But derby has allowed me to easily get my feet wet in the database world without any major hassles.
I love the fact that it's written in java. Since I only have a Linux box to develop on, but must deploy for a Windows environment, simply being able to package derby.jar with my build and knowing that it will work is very handy.
Plus, OpenOffice and Netbeans both support it, which is handy.