I have read about Oracle 11g Active Data Guard and pretty impressed with the enhancements done in the Data Guard technology.
Although, I haven't tested those enhancements, but, looks very useful.
Following are the few notable enhancements:
1. Enabling physical standby database in read only mode while all the changes from the production database are being applied to it. This feature resemblance to logical standby database techniques in previous Oracle versions.
2. Snapshot Standby - A snapshot Standby is open for read-write which can be ideally suited for test environments, able to process transactions independently of the primary database. At the same time, it maintains protection by continuing to receive data from the production database, archiving it for later use.
Using a single command discard changes made while read-write mode and quickly resynchronizes the standby with the primary database.
I really need to do a test case on these enhancement in order to find out whether it really work the way Oracle says or any bugs associates with them.
For more information, read the following pdf document which is available at Oracle site.