Who should use OpenJDK?
Any new J2SE 6 development or deployment work you are doing is a perfect candidate for using OpenJDK 6. As an example this website uses OpenJDK 6 because Subetha 2.0 and Resin 4.0 both require JDK 1.6 (and we wanted to eat our own dog food).
OpenJDK is THE Java distribution that all the Linux vendors are using going forward. Also, Sun/Oracle likes to throw in lots of extra gunk, that I don't think is best-of-breed, along with their proprietary J2SE binaries such as Java DB, Netbeans and GlassFish.
Additionally, Sun recently end-of-life'd J2SE 5 and you are required to pay for a Java For Business license if you want to "ensure uninterrupted and immediate access to critical fixes". This is typical vendor lock-in behavior that you and your clients won't be subject too if you stick to Free And Open Source (FOSS) such as OpenJDK.